Issue: SPR-17086

Issue: SPR-16966

Issue: SPR-16966

Fix checkstyle violations for javadoc.

Issue: SPR-16968

Issue: SPR-16898

Issue: SPR-16218

This commit adds
AbstractMessageReaderArgumentResolver#readBody and
AbstractMessageWriterResultHandler#writeBody variants
which allow to pass the actual MethodParameter in order
to perform proper annotation-based hint resolution with
nested generics, for example with HttpEntity.

Issue: SPR-16098

Issue: SPR-16172

Issue: SPR-15910

Issue: SPR-15731

Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.

Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.

Issue: SPR-15540

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.

ServerHttpMessage[Reader|Writer] are now expected for WebFlux,
annotated controller endpoint and subsequently the instanceof checks
for HttpMessage[Reader|Writer] vs ServerHttpMessage[Reader|Writer] can
be removed from AbstractMessageReaderArgumentResolver and
AbtractMessageWriterResultHandler.

Prior to this commit, WebFlux would look at the handler method
annotations (`@ResponseStatus`) for each handler execution, even calling
the expensive `synthesizeAnnotation`.

This commit moves this logic to the InvocableHandlerMethod so that this
executed once at instantiation time and for all result handlers.

Issue: SPR-15227

Issue: SPR-15190

Simplify getAdapterFrom/To into a single getAdapter method that looks
for an exact match by type first and then isAssignableFrom.

Also expose shortcut methods in ReactiveAdapter to minimize the need
for access to the ReactiveTypeDescriptor.

Issue: SPR-14902

If the body class is not resolvable from the return type and there is
a body instance we now fall back on the class of the body instance.

Issue: SPR-14877

This commit adds support for `@ResponseStatus` annotations on reactive
controller methods. `HandlerResultHandler`s implementations now
set the status on the `ServerWebExchange`, if and only if the
invocation of the controller method succeeded.

Issue: SPR-14830

This commit makes it possible, in addition to provide hints, to
perform additional operations with the request and the response
at ServerHttpMessageReader/Writer level.

AbstractServerHttpMessageReader/Writer now provide
convenient beforeRead/beforeWrite abstract methods for such need.

Issue: SPR-14557

Issue: SPR-14693

Issue: SPR-14557

Issue: SPR-14159

When using the ConversionService to check and bridge to and from
reactive types we now generallly provide the full type information
available from method signatures. However that full type information
is not always necessary such as when we perform additional checks on
the generics of the reactive type (e.g. Mono<ResponseEntity>).

This allows us to switch to use DefaultFormattingConversionService
instead of GenericConversionService while also ensuring that the
CollectionToObjectConverter doesn't think it can convert List<?> to
any reactive type.

The ObjectToObjectConverter can also interfere because it is smart
enough to find the "from(Publisher<?>)" method on Flux and Mono.
To make up for that on the response side we now check if a type
is assignable to Publisher first in which case it is a simple cast.

In turn that means we don't need a PublisherToFluxConverter which can
be problematic in its own right because it can convert from Mono to
Flux which technically doesn't lose data but switches stream semantics.

Issue: #124, #128

This commit ensures stream semantics (Flux vs Mono) are adhered to also
on the target side.

Introduce separate test classes for each base class in the hierarchy
above @ResponseBody and ResponseEntity result handlers.

Also start porting existing unit test cases for @ResponseBody and
ResponseEntity return value handlers.

The ContentNegotiatingResultHandlerSupport base class encapsulates the
logic for content negotiation needed for both @ResponseBody and view
resolution result handling.