The key in CachingResourceResolver now includes the "Accept-Encoding"
request header cleaned to exclude "*", "identity", and parameters, and
also sorted alphabetically.

For encoded resources the response now includes a response header with
"Vary: Accept-Encoding".

Issue: SPR-16381

Issue: SPR-16862

The new EncodedResourceResolver is a generalized version of
GzipResourceResolver that can be configured to support different
content codings, by "br" and "gzip".

GzipResourceResolver is now deprecated.

Issue: SPR-16381

Improve how HTTP message writers are obtained for general use vs for
multipart requests.

Functionally equivalent updates to package private classes to improve
the code and make it easier to understand.

This commit makes it possible to pass attributes from the WebSession of
a handshake request to the WebSocketSession, by configuring a
Predicate<String> on HandshakeWebSocketService.

Issue: SPR-16212

Issue: SPR-16212

Issue: SPR-16844

Issue: SPR-16531

Issue: SPR-16843

Issue: SPR-16831

Issue: SPR-16830

Issue: SPR-16820

Issue: SPR-16829

Issue: SPR-16820

After the recent changes to expose configuring TcpOperations, it no
longer makes sense to automatically log the relayHost/Port since that's
mutually exclusive with a custom TcpOperations.

Instead we delegate to TcpOperations.toString().

Issue: SPR-16801

Issue: SPR-16804

Issue: SPR-16531

Issue: SPR-16829

Issue: SPR-16811

Issue: SPR-16816

Issue: SPR-16826

Issue: SPR-16803

Issue: SPR-16429

Issue: SPR-16823

Issue: SPR-16303

Issue: SPR-16411

Issue: SPR-16804

Issue: SPR-16805