Provide DataDog veneer for generated ApiClient

[jdimeo]

Note:
If you have a feature request, you should contact support so the request can be properly tracked.

Is your feature request related to a problem? Please describe.
Right now the "raw" generated ApiClient is difficult to configure and use. There can be many ApiClient classes on the classpath (I have dozens) from various APIs.

Describe the solution you'd like

public class DataDogClient extends ApiClient {
    ...
}

and then we are instructed to use this as the entrypoint.

This additionally allows for avoiding code smells, instead of being forced to set DD_SITE env var or knowing the internals of the server variable naming, this veneer client can provide a setter like setSite("datadoghq.com") that configures the server variables for us, hiding those implementation details.

Describe alternatives you've considered
Totally optional- can keep rolling with what you have now.

Additional context
This helps new developers begin using this great client more quickly and more safely.