See gh-23999

Before this change an @MessageMapping could be matched to any RSocket
interaction type, which is arguably too flexible, makes it difficult to
reason what would happen in case of a significant mismatch of
cardinality, e.g. request for Fire-And-Forget (1-to-0) mapped to a
method that returns Flux, and could result in payloads being ignored,
or not seen unintentionally.

This commit checks @ConnectMapping method on startup and rejects them
if they return any values (sync or async). It also refines each
@MessageMapping to match only the RSocket interaction type it fits
based on the input and output cardinality of the handler method.
Subsequently if a request is not matched, we'll do a second search to
identify partial matches (by route only) and raise a helpful error that
explains which interaction type is actually supported.

The reference docs has been updated to explain the options.

Closes gh-23999

The new annotation helps to differentiate the handling of connection
level frames (SETUP and METADATA_PUSH) from the 4 stream requests.

Closes gh-23177