Issue: SPR-15318

Issue: SPR-15367

Issue: SPR-14925

This commit changes the `HandlerStrategies` builder to use
`ServerCodecConfigurer` for configuring Decoder|Encoder and
HttpMessage[Reader|Writer]. Note that the configurer is not exposed
directly, but wrapped so that `HandlerStrategies` can keep on returning
`this` for a proper builder pattern.

This commit also removes the HandlerStrategies methods that take an
application context, as it was too naive approach to simply look up
every message reader and writer in the context.

Issue: SPR-15415

- Renamed `defaultCodec` to `defaultCodecs`, and `customCodec` to
`customCodecs`
- Added `@Override` annotations where necessary
- Fixed non-parameterized usage for parameterized types.

This commit makes CodecException handling consistent between functional
and annotation-based APIs. It now returns by default 4xx status code
for decoding error and 5xx for encoding error + print the error reason
in logs without the full stack trace in both variants.

Issue: SPR-15355

Issue: SPR-15227

The java.util.Optional wrapper should not affect the support for
"request param" arguments with or without the annotation as it
works on the Spring MVC side.

Issue: SPR-15211

This commit uses the newly introduced
`PathPattern.getPathRemaining(String)` in the functional web framework.
With this change, all path predicates can be used for nested router
functions, so the `pathPrefix` predicate is no longer required and has
been removed.

Issue: SPR-15336

This commit introduces support for the server-side methods on
HttpMessageReader and HttpMessageWriter. It does so by introducing an
Optional ServerHttpRequest in BodyInserter.Context, and an Optional
ServerHttpResponse in BodyExtractor.Context. On the client-side, these
optionals return Optional.empty(); on the server-side, they return the
respective server-side messages.

Issue: SPR-15370

Closes gh-1370

This change allows the functional WebFlux API to support natively
reactive types and also makes it possible for View implementations to
disable async attributes resolution if they want for example take
advantage of stream rendering.

It also makes AbstractView#getModelAttributes() asynchronous.

Issue: SPR-15368

Replace the overloaded "retrieve" methods with a single retrieve() +
ResponseSpec exposing shortcut methods (bodyToMono, bodyToFlux)
mirroring the ClientResponse shortcuts it delegates to.

Unlike exchange() however with retrieve() there is no access to other
parts of ClientResponse so ResponseSpec exposes additional shortcuts
for obtain ResponseEntity<T> or ResponseEntity<List<T>>.

Issue: SPR-15294

This commit makes a change to WebClient in oder to facilitate getting
the response body as a `Mono<Object>` or `Flux<Object>` without having
to deal with `ClientResponse`.

Specifically, this commit:

 - Adds `RequestHeaderSpec.retrieve` methods, next to `exchange`, that
 return the response body (and not a `ClientResponse`). Two convenience
 methods return the response body as `Mono` or `Flux`.
 - Adds ClientResponse.toRequestEntity to convert the ClientResponse
 into a RequestEntity.

Issue: SPR-15294

The main purpose is to specify nullability.

The ServerCodecConfigurer is now passed all the way into the
RequestMappingHandlerAdapter which automatically enables the same
defaults even without the Java config and provides extra flexibility
in passing message codec configuration for the future.

Replace the List<HandlerMethodArgumentResolver> with a dedicated
configurer that currently has one method accepting custom resolver
registrations.

This commit makes changes to WebClient and WebTestClient in oder to
limit setting the body according to HTTP method and also to facilitate
providing the request body as Object.

Specifically, this commit:

 - Moves methods that operate on the request body to a RequestBodySpec
 in both WebClient and WebTestClient, and rename them to `body`.
 These methods now just *set* the body, without performing
 an exchange (which now requires an explicit exchange call).
 - Parameterizes UriSpec in both WebClient and WebTestClient, so that
 it returns either a RequestHeadersSpec or a RequestBodySpec.

Issue: SPR-15394

ControllerMethodResolver now also encapsulates initialization, storage,
and use of HandlerMethodArgumentResolver's by annotated method type.

Extract controller method caches including associated code and
discovery of @ControllerAdvice components into a separate, package
private helper class (ControllerMethodResolver).

Issue: SPR-15247

Issue: SPR-15247

This commit folds ServerHttpMessage[Reader|Writer] into its parent
HttpMessage[Reader|Writer] with the server methods pre-implemented
by default to be simple pass-through delegates.

Issue: SPR-15247

This commit changes `ResourceTransformerSupport` to look for the
`ResourceUrlProvider` in the current request if none is configured on
the resource transformer itself.

Issue: SPR-15369

Prior to this commit, the `VersionResourceResolver` implementations of
`resolveUrlPathInternal` would delegate to the resolver chain but would
never use the give result if the current request didn't match a
configured version strategy pattern.

This is a problem if the resolver supposed to resolve the resource path
is configured after a `VersionResourceResolver` in the resolver chain;
this means that other resolver never gets to participate in the result
of the chain.

Issue: SPR-15372

Includes a new overloaded ModelAndView constructor with an HttpStatus argument, as well as a HandlerMethodArgumentResolverSupport refactoring (revised checkParameterType signature, actually implementing the HandlerMethodArgumentResolver interface).

Issue: SPR-15199

Follow-up to:
https://github.com/spring-projects/spring-framework/commit/3d68c496f15c80a75711480a569cda1f7c64dc91

StringDecoder can be created in text-only vs "*/*" mode which in turn
allows a more intuitive order of client side decoders, e.g. SSE does
not have to be ahead of StringDecoder.

The commit also explicitly disables String from the supported types in
Jackson2Decoder leaving it to the StringDecoder in "*/*" mode which
comes after. This does not change the current arrangement since the
the StringDecoder ahead having "*/*" picks up JSON content just the
same.

From a broader perspective this change allows any decoder to deal with
String if it wants to after examining the content type be it the SSE
or another, custom decoder. For Jackson there is very little value in
decoding to String which works only if the output contains a single
JSON string but will fail to parse anything else (JSON object/array)
while StringDecoder in "*/*" mode will not fail.

Issue: SPR-15374