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

See gh-23417

Closes gh-23882

See gh-23882

Take a slice of the metadata if not using composite metadata to allow
reading it multiple times. For composite metadata this is not an isuse
as it is ensured by the underlying RSocket Java API.

This commit renames the `metadataExtractors` method to something that is
closer to the actual intent: addming new metadata extractors against the
registry given as a parameter of the `Consumer`.

The method is renamed to `metadataExtractorRegistry`.

See gh-23649

This commit makes it possible to send requests without
requiring to call data(Mono.empty()). It introduces a
dedicated MetadataSpec interface and merge ResponseSpec
into RequestSpec for more flexibility.

Closes gh-23649

Prior to this commit, customizing the extraction of RSocket metadata
from frames would require developers to override the default
`MetadataExtractor` while configuring `RSocketStrategies`.
This touches on many infrastructure parts, whereas the goal is just to
configure an extra metadata entry extractor using already configured
codecs.

This commit adds a way to register metadata entry extractors on the
`RSocketStrategies` builder with a `Consumer`-based API.

Closes gh-23645

See gh-23539
Co-authored-by: NSam Brannen <sbrannen@pivotal.io>

See gh-23539

Closes gh-23513

This allows encoding of metadata values externally via some
existing API for encoding RSocket metadata, rather than relying on
registered Encoders.

Also remove explicit checks for DataBuffer since those are supported
through the DataBufferEncoder (registered by default).

See gh-23513

Closes gh-23137

For public use, these constants aren't ideally exposed through an SPI
like MetadataExtractor, and there isn't any other obvious place either.
In practice the only public API where these can be passed in is
RSocketRequester and RSocketMessageHandler both of which already
default to composite metadata anyway, leaving only the routing MimeType
to be used potentially but much less likely.
Due to existence of similar constants in the RSocket itself, i.e.
WellKnownMimeType, we can get by internally too without declaring
MimeType constants from a central place.

take advantage of the symmetrical SocketAcceptor methods now available
on RSocketFactory for both client and server side.

Deprecate all mutation methods in `MethodParameter` in favor of factory
methods that return a new instance. Existing code that previously relied
on mutation has been updated to use the replacement methods.

Closes gh-23385

Expecting data first and metadata second aligns better with the single
arg variant that accepts data. This is also consistent with the
RSocket API in the create methods of ByteBufPayload and DefaultPayload.

Closes gh-23358

Closes gh-23368

To be re-used also for creating metadata for the setup payload.

See: gh-23368

Use rsocketStrategies field with mutate() to ensure consistency
with internal state.

Remove transparent initialization of decoders in MetadataExtractor
and expect them to be set to avoid unintended side effects.