JCLOUDS-633: Support passing bearer token directly for OAuth2

https://issues.apache.org/jira/browse/JCLOUDS-633
Work in progress, I will be adding some tests soon but wanted to expose this for review.
> I don't quite understand the question

Sorry, I wasn't very clear there. I meant: "If we now have the provider, does the `@ImplementedBy` annotation still make sense?"