Issue: SPR-15758

Explicit typing for older jvm compilation in BodyExtractor

This commit adds support for Void response body types in the WebClient,
both when using `exchange` with a response.bodyToMono(Void.class), as
well as using `retrieve` with `toEntity(Void.class)`.

Issue: SPR-15679

This commit changes the use of `ResolvableType` to
`ParameterizedTypeReference` in all public-facing WebFlux APIs. This
change removes the necessity for providing the parameterized type
information twice: once for creating the `ResolvableType`, and once for
specifying a `BodyExtractor`.

Issue: SPR-15636

This commit changes all methods that return `Supplier<Stream<T>` to
return `List<T>` instead. This includes the HandlerStrategies, but also
BodyInserter.Context, BodyExtractor.Context, and ServerResponse.Context.

The reason for this change is that most of the strategies have some sort
of order, where more specific message readers - for instance - come
before generic ones. Such an ordering can only be enforced through a
List.

Issue: SPR-15578

This commit adds a `toParts` method in `BodyExtractors`, returning a
BodyExtractor<Part>.

Issue: SPR-14546

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

This commit reverses 9efa976d, and adds
code comments to highlight why some Body[Inserter|Extractor] instances
returned from Body[Inserters|Extractors] use
ServerHttp[Request|Response] instead of using
Reactive[Input|Output]Message.

Issue: SPR-15190

Moved UnsupportedMediaTypeException next to Body[Inserters|Extractors],
as that is where they are used. This move should have been done as part
of the inserter and extractor move, but was forgotten.

 - Added BodyInserter for MultiValueMap form data in BodyInserters
 - Added BodyExtractor to MultiValueMap in BodyExtractors

Issue: SPR-15144

The most common use case is specifying JSON views.

ServerResponse.BodyBuilder#hint(String, Object) allows to
specify response body serialization hints.

ServerRequest#body(BodyExtractor, Map) allows to specify
request body extraction hints.

Issue: SPR-15030

Added a BodyExtractor for Flux<DataBuffer>, and a BodyInserter for
Publisher<DataBuffer>

Issue: SPR-14918

This commit moves the web.reactive.function.[BodyInserter|BodyExtractor]
to http.codec, so that they can be used from the client as well.

Furthermore, it parameterized both inserter and extractor over
ReactiveHttpOutputMessage and ReactiveHttpInputMessage respectively, so
that they can be limited to only be used on the client or server.

Issue: SPR-14557

This commit refactors the response body extraction logic into a separate
function: BodyExtractor. Standard populators can be found in
BodyExtractors.