Request factory and raw JWKS copy

This release contains two new features.

The first is a new field on keyfunc.Options for a request factory. This allows the package user to create the *http.Request used to get the JWKS from the remote resource. This serves multiple use cases, such as attaching a User-Agent via an HTTP header. This is thanks to a new contributor @mgsh!

The second is a new method to get the latest raw JWKS []byte returned from the remote resource.

A few other updates:

• Tests updated to use Fatalf instead of a combination of Errorf and FailNow.
• Removed usage of err.Error() when using string formatting.
• Using logFmt string formatting constant for log statements in tests.