The WebSocketHander adapters are now neutral for client vs server-side
use with the adapters for RxNetty and Reactor Netty (server-side only)
completely removed.

A new HandshakeInfo carries information about the handshake including
URI, headers, and principal from the upgrade strategy, to the adapter,
and then to the session.

WebSocketSession exposes the HandshakeInfo as well reducing its overall
number of methods.

Expose bufferFactory() at the WebSocketSession level for creating
payloads like ReactiveHttpOutputMessage does.

Promote getId(), getUri(), and bufferFactory() to the base class
WebSocketSessionSupport.

suspend/resumeReceiving in the AbstractListenerWebSocketSession are
now abstract methods. In Tomcat/Jetty these methods are no-op
implementations that are then coupled with a buffering strategy via
Flux#onBackpressureBuffer. In Undertow they rely on flow control for
receiving WebSocket messages.

Issue: SPR-14527

Dropped "Support" from the name since it not only provides support
methods but actually implements WebSocketSession.

Renamed inner classes:
WebSocketMessagePublisher -> WebSocketReceivePublisher
WebSocketMessageProcessor -> WebSocketSendProcessor

Add protected getter for sendProcessor.

Reduce scoping:
WebSocketReceivePublisher -> private
WebSocketSendProcessor -> protected
WebSocketSendProcessor#setReady -> public (class is still protected)

A few more method name alignments and Javadoc updates.

Issue: SPR-14527

Issue: SPR-14527

Issue: SPR-14527

Issue: SPR-14527