Split up NimbusOAuth2UserService

[rwinch]

Summary

Currently NimbusOAuth2UserService supports custom users, OIDC Users, and OAuth Users. It would be better if we could decouple this into 3 separate services and have a generic delegating OAuth2UserService. This simplifies the class, simplifies expanding other use cases, and ensures users can leverage only some of the functionality.

NOTE: Splitting up also allows JWT to be optional dependency if using something like GitHub which doesn't use JWT