Spun out from #6219.

We need the ability to apply a UserDetailsChecker to the OAuth2 Resource Server flows.

My initial thoughts on implementation would be to have converters (i.e. Converter<Jwt, AbstractAuthenticationToken> & Converter<Jwt, Mono<AbstractAuthenticationToken>>) that can take an underlying Converter and apply a UserDetailsChecker to it before returning the AbstractAuthenticationToken/Mono<AbstractAuthenticationToken>.

That way in the configuration DSL a consumer could choose to have this ability or not via something like

http.oauth2ResourceServer()
  .jwt()
    .jwtAuthenticationConverter(new ReactiveJwtPostAutneticationChecksAuthenticationConverter(userDetailsService));

I'm open to discussing the correct approach on how best to integrate it and am happy to supply a PR for the correct approach.

If you'd rather me submit a PR with what I have and discuss the approach on the PR thats fine too.