This commit ensure that null-safety is consistent between
getters and setters in order to be able to provide beans
with properties with a common type when type safety is
taken in account like with Kotlin.

It also add a few missing property level @Nullable
annotations.

Issue: SPR-15792

Issue: SPR-15755

Issue: SPR-15720

Issue: SPR-15791

Since there is no reason for an exchange to ever complete without a
ClientResponse I've added a switchIfEmpty check at the WebClient level.

Also, temporarily a second check closer to the problem in the
ReactorClientHttpConnector suggesting a workaround and providing a
reference to the Reactor Netty issue #138.

Issue: SPR-15784

Issue: SPR-15785

Issue: SPR-15773

Collapse the base interface VersionPathStrategy into its extension
VersionStrategy and then turn the prefix nad fliename based
implementations into abstract base classes (vs delegate strategies).

It is simpler to have one VersionStrategy hierarchy vs that plus a
separate VersionPathStrategy as a delegate. In practice each
VersionStrategy is suited to be prefix or filename based. Also none
of our code cares about the distinction between those two interfaces.

Issue: SPR-15787

Since `PathPattern.combine` now returns another `PathPattern` instance
(it was previously returning a String instance), we can now safely
remove the parser instance included in `PatternsRequestCondition`.

Issue: SPR-15663

Issue: SPR-15773

Issue: SPR-15775

Issue: SPR-15748

Currently ResourceEncoder and ResourceRegionEncoder use DataBufferUtils
to read resource with an AsynchronousFileChannel if it is a file or
otherwise fallback on getting the channel from the resource.

The same is now required in other places where a Resource needs to be
read and is also generally useful.

Issue: SPR-15773

Issue: SPR-15571

Use copy constructor to refresh a session with lastAccessTime and a
save function referencing the current exchange. As a result both fields
are now final and ConfigurableWebSession is no longer needed.

This commit changes the write methods to return `Flux<DataBuffer>`
instead of `Mono<Void>`, giving access to the original buffers,
so that they can decided whether the buffers need to be closed or not.

Issue: SPR-15726

This commit adds an overloaded write method to `DataBufferUtils`. There
are three parameter variants: `OutputStream`, `WritableByteChannel`, and
`AsynchronousFileChannel`.

Issue: SPR-15726

Issue: SPR-15643

Issue: SPR-15770

Issue: SPR-15769

Issue: SPR-15720

Issue: SPR-15758

This commit introduces overloaded variants of `bodytoMono`,
`bodyToFlux`, `toEntity`, and `toEntityList` that take a
`ParameterizedTypeReference`. It also adds similar methods to
`WebClient.ResponseSpec`.

Issue: SPR-15725

Issue: SPR-15720

This binary format more efficient than JSON should be useful for server
to server communication, for example in micro-services use cases.

Issue: SPR-15424

Issue: SPR-15757

Issue: SPR-15766
(cherry picked from commit c9ffa641)