Support @RSocketExchange for responding

[OlgaMaciaszek]

Fixes gh-29240.

[rstoyanchev]

For the documentation, while it is logical to mention @RSocketExchange alongside @MessageMapping, I'm not sure I like the result. It gets repetitive and becomes a distraction.

I think it would be better to create a dedicated section under "Annotated Responders" called @RSocketExchange, probably right after @MessageMapping where we can properly explain all that needs explaining. For a start what it is in contrast to @MessageMapping (with a reference to the RSocket Service section), that it is is supported for responding as an alternative to @MessageMapping so you can essentially use it for requesting and responding, that it supports very similar but slightly different method signature as it needs to remain neutral to requester vs responder, and so on. The RSocket Service proxy section could also mention that the annotation is supported for responding to make sure that it's discoverable. Other than that I don't think we need to mention it everywhere alongside @MessageMapping. This will become even more obvious with the same changes for @HttpExchange since we have much more extensive documentation that mentions @RequestMapping many times.