Support @RSocketExchange for annotated responders

[CoderYellow]

It's better to addopt unifed annotation contract for both server and client side. And I think the RSocket Interface annotation is more powerful and meaningful

[rstoyanchev]

We can enhance Annotated Responders to support @RSocketExchange instead of @MessageMapping, which would allow using the same interface to make and to handle requests.

The semantics overlap to a large degree. These are the differences I see:

• Metadata Object, followed by MimeType -- supported for @RSocketExchange but not for @MessageMapping; we can add arguments resolvers to support it
• RSocketRequester argument -- this is a responder-only concept, and therefore not applicable with @RSocketExchange
• Synchronous return type -- for @MessageMapping we assume non-blocking handling (no I/O essentially), while for requests with @RSocketExchange it is a way to block for the result; we can proactively switch threads when such methods are used for handling with @RSocketExchange

[CoderYellow]

yes, something like MVC annotation, it's better have rescoket team to make a common standard like JAX-RS annotation, so that other framework can implement it too

[OlgaMaciaszek]

We are going to experiment with this for 6.1.

[rstoyanchev]

Superseded by #30936.