[Internal] Move package credentials in config (#1115)

## What changes are proposed in this pull request?

This PR moves the `credentials` package in `config`, thus centralizing
the credentials logic in a single place.

The goal of this change is to prepare the current SDK-Beta to facilitate
migration to the new SDK that is currently being developed.

This PR introduces a breaking change by changing the import paths to
`CredentialsProvider`. However, we believe that the impact is fairly
contained as (i) `CredentialsProvider` is mostly internally, and (ii)
the breaking change can be addressed by updating the import path.

## How is this tested?

Through normal unit test and integration tests.

config/api_client.go

@@ -9,7 +9,7 @@ import (
 	"time"
 
 	"github.com/databricks/databricks-sdk-go/apierr"
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/httpclient"
 	"github.com/databricks/databricks-sdk-go/useragent"
 )

config/auth_azure_cli.go

@@ -11,7 +11,7 @@ import (
 
 	"golang.org/x/oauth2"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/logger"
 )
 

config/auth_azure_client_secret.go

@@ -8,7 +8,7 @@ import (
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/clientcredentials"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/logger"
 )
 

config/auth_azure_github_oidc.go

@@ -6,7 +6,7 @@ import (
 	"fmt"
 	"time"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/httpclient"
 	"github.com/databricks/databricks-sdk-go/logger"
 	"golang.org/x/oauth2"

config/auth_azure_msi.go

@@ -8,7 +8,7 @@ import (
 	"net/http"
 	"time"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/httpclient"
 	"github.com/databricks/databricks-sdk-go/logger"
 	"golang.org/x/oauth2"

config/auth_basic.go

@@ -6,7 +6,7 @@ import (
 	"fmt"
 	"net/http"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 )
 
 type BasicCredentials struct {

config/auth_databricks_cli.go

@@ -10,7 +10,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/logger"
 	"golang.org/x/oauth2"
 )

config/auth_default.go

@@ -5,7 +5,7 @@ import (
 	"errors"
 	"fmt"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/logger"
 )
 

config/auth_gcp_google_credentials.go

@@ -6,7 +6,7 @@ import (
 	"io/ioutil"
 	"os"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/logger"
 	"golang.org/x/oauth2/google"
 	"google.golang.org/api/idtoken"

config/auth_gcp_google_id.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"fmt"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/logger"
 	"golang.org/x/oauth2"
 	"google.golang.org/api/impersonate"

config/auth_m2m.go

@@ -8,7 +8,7 @@ import (
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/clientcredentials"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/httpclient"
 	"github.com/databricks/databricks-sdk-go/logger"
 )

config/auth_metadata_service.go

@@ -8,7 +8,7 @@ import (
 	"net/url"
 	"time"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/httpclient"
 	"github.com/databricks/databricks-sdk-go/logger"
 	"golang.org/x/oauth2"

config/auth_pat.go

@@ -5,7 +5,7 @@ import (
 	"fmt"
 	"net/http"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 )
 
 type PatCredentials struct {

config/config.go

@@ -13,7 +13,7 @@ import (
 
 	"github.com/databricks/databricks-sdk-go/common"
 	"github.com/databricks/databricks-sdk-go/common/environment"
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/httpclient"
 	"github.com/databricks/databricks-sdk-go/logger"
 	"golang.org/x/oauth2"

config/credentials/credentials.go

@@ -0,0 +1,68 @@
+package credentials
+
+import (
+	"net/http"
+
+	"golang.org/x/oauth2"
+)
+
+// CredentialsProvider is an interface for providing credentials to the client.
+// Implementations of this interface should set the necessary headers on the request.
+type CredentialsProvider interface {
+	// SetHeaders sets the necessary headers on the request.
+	SetHeaders(r *http.Request) error
+}
+
+type credentialsProvider struct {
+	setHeaders func(r *http.Request) error
+}
+
+func (c *credentialsProvider) SetHeaders(r *http.Request) error {
+	return c.setHeaders(r)
+}
+
+func NewCredentialsProvider(visitor func(r *http.Request) error) CredentialsProvider {
+	return &credentialsProvider{
+		setHeaders: visitor,
+	}
+}
+
+// OAuthCredentialsProvider is a specialized CredentialsProvider uses and provides an OAuth token.
+type OAuthCredentialsProvider interface {
+	CredentialsProvider
+	// Token returns the OAuth token generated by the provider.
+	Token() (*oauth2.Token, error)
+}
+
+type oauthCredentialsProvider struct {
+	setHeaders func(r *http.Request) error
+	token      func() (*oauth2.Token, error)
+}
+
+func (c *oauthCredentialsProvider) SetHeaders(r *http.Request) error {
+	return c.setHeaders(r)
+}
+
+func (c *oauthCredentialsProvider) Token() (*oauth2.Token, error) {
+	return c.token()
+}
+
+func NewOAuthCredentialsProvider(visitor func(r *http.Request) error, tokenProvider func() (*oauth2.Token, error)) OAuthCredentialsProvider {
+	return &oauthCredentialsProvider{
+		setHeaders: visitor,
+		token:      tokenProvider,
+	}
+}
+
+// OAuthToken represents an OAuth token as defined by the OAuth 2.0 Authorization Framework.
+// https://datatracker.ietf.org/doc/html/rfc6749
+type OAuthToken struct {
+	// The access token issued by the authorization server. This is the token that will be used to authenticate requests.
+	AccessToken string `json:"access_token"  auth:",sensitive"`
+	// Time in seconds until the token expires.
+	ExpiresIn int `json:"expires_in"`
+	// The scope of the token. This is a space-separated list of strings that represent the permissions granted by the token.
+	Scope string `json:"scope"`
+	// The type of token that was issued.
+	TokenType string `json:"token_type"`
+}

examples/custom-auth/main.go

@@ -10,7 +10,7 @@ import (
 
 	"github.com/databricks/databricks-sdk-go"
 	"github.com/databricks/databricks-sdk-go/config"
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"github.com/databricks/databricks-sdk-go/service/compute"
 )
 

httpclient/oauth_token.go

@@ -5,7 +5,7 @@ import (
 	"net/http"
 	"time"
 
-	"github.com/databricks/databricks-sdk-go/credentials"
+	"github.com/databricks/databricks-sdk-go/config/credentials"
 	"golang.org/x/oauth2"
 )