This commit allows single-value async producers for the values of
metadata entries in both the SETUP and for requests. The same is also
enabled for data in the SETUP frame.

Close gh-23640

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

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

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.

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

See: gh-23368

Closes gh-23310

The responding side now relies on a new MetadataExtractor which decodes
metadata entries of interest, and adds them to an output map whose
values are then added as Message headers, and are hence accessible to
controller methods.

Decoded metadata entry values can be added to the output map one for
one, or translated to any number of values (e.g. JSON properties),
as long as one of the resulting pairs has a key called "route".

On the requesting side, now any metadata can be sent, and a String
route for example is not required to be provided explicitly. Instead
an application could create any metadata (e.g. JSON properties) as long
as the server can work out the route from it.

The commit contains further refinements on the requesting side so that
any mime type can be used, not only composite or routing metadata, e.g.
a route in an "text/plain" entry.

Closes gh-23157

Replace the Publisher argument in RequestSpec's
data(Publisher, Class<T>) and
data(Publisher, ParameterizedTypeReference<T>) methods with Object thus
allowing any reactive type known to the ReactiveAdapterRegistry to be
passed in directly rather than adapted to Publisher first.

This commit removes the MessageHandlerAcceptor sub-class of
RSocketMessageHandler, and rather than implementing directly the
contracts for RSocket client and server acceptors, RSocketMessageHandler
now exposes clientAcceptor() and serverAcceptor() methods that return
the required adapter instances.

This provides better separation between the RSocketMessageHandler and
the RSocket adapter code, and also avoids implementing generic
interfaces like the BiFunction required for the client acceptor.

Closes gh-22798

Closes gh-22782

See gh-22782

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

# Fixed URLs

## Fixed Success
These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

* [ ] http://www.apache.org/licenses/ with 1 occurrences migrated to:
  https://www.apache.org/licenses/ ([https](https://www.apache.org/licenses/) result 200).
* [ ] http://www.apache.org/licenses/LICENSE-2.0 with 6625 occurrences migrated to:
  https://www.apache.org/licenses/LICENSE-2.0 ([https](https://www.apache.org/licenses/LICENSE-2.0) result 200).

- Integration tests run with zero copy configuration.
- RSocketBufferLeakTests has been added.
- Updates in MessagingRSocket to ensure proper release

See gh-21987

See gh-21987

See gh-21987