Improve customization of `DefaultOAuth2UserService` to handle other content types

Expected Behavior

DefaultOAuth2UserService can be extended to e.g. allow for custom body parsing to handle application/jwtfor signed and/or encrypted UserInfo Response.

Rough draft:

public class CustomOAuth2UserService extends DefaultOAuth2UserService {

   @Override
    protected ResponseEntity<Map<String, Object>> getResponse(OAuth2UserRequest userRequest, RequestEntity<?> request) {
      // Custom code to handle requests that aren't simple application/json
      return ...;
     }
}

We are open for other solutions as well and happy to contribute, if that's something you see worth it as addition to spring-security.

Current Behavior

DefaultOAuth2UserService has to be copied and "rewritten" - because getResponse() is called inside loadUser(OAuth2UserRequest userRequest) which forces us to re-create the whole loadUser(OAuth2UserRequest userRequest) method.

https://github.com/spring-projects/spring-security/blob/a325216f19277d4191c97afee1c66f82d056f9dc/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/userinfo/DefaultOAuth2UserService.java#L88-L117

Context

Related to #9583