Add sample for Opaque Token endpoint that returns a JWT

[jzheaux]

Related to spring-projects/spring-security#15467

It may be possible to configure Spring Authorization Server to return a JWT as part of the introspection endpoint. Given that, it would be nice to demonstrate how to create a custom OpaqueTokenIntrospector that returns the verified JWT.

[franticticktick]

Hi @jzheaux. I will work on this issue this week.