Make ResourceHandlerRegistrationCustomizer from WebFluxConfig visible to user code

[dsyer]

We made some changes to improve the re-usability of autoconfig classes in #13490. This one slipped through the net somehow - you still can't create an instance of the public class WebFluxConfig from outside its own package because of a private interface ResourceHandlerRegistrationCustomizer. The same issue probably is mirrored in Mvc autoconfiguration.