Also revises copyToByteArray/String in FileCopyUtils/StreamUtils for lenient null handling.

Issue: SPR-15329

Issue: SPR-15332

Issue: SPR-15350

Issue: SPR-15350

This commit fixed the exception that occurs when a larger file is
transfered using zero-copy on Undertow.

Issue: SPR-15343

This commit introduces support for the ResponseStatusException in the
functional web framework.

Issue: SPR-15344

ExchangeMutatorWebFilter now supports multiple mutator registrations
for the same request header id by creating a single composed function.

Issue: SPR-15250

CompositeHttpHandler is public and called ContextPathCompositeHandler.

Also an overhaul of the Javadoc on HttpHandler, WebHttpHandlerAdapter,
and ContextPathCompositeHandler.

Extract CompositeHttpHandler to a package private class and add direct
support via `HttpHandler.of(...)`. This removes the need for the
`HttpHandlerAdapterSupport` class.

The goal is to have better consistency between Java
and Kotlin functional Web API.

All method argument resolvers now explicitly check for the presence
of a reactive type wrapper and reject it where not expected.

Issue: SPR-15297

Issue: SPR-15333

Issue: SPR-15332

Issue: SPR-14432

Issue: SPR-15340

Switch from extension to delegation model, i.e. delegating to
InvocableHandlerMethod, so that only sync invocation is exposed and
only resolvers of type SyncHandlerMethodArgumentResolver are allowed
to be configured in a cleaner fashion.

Issue: SPR-15250

This commit adds a common base class for server-less setup with the
option to configure a transformation function on the
ServerWebExchange for every request.

The transformation is applied through a WebFilter. As a result the
RouterFunction setup is now invoked behind a DispatcherHandler with
a HandlerMapping + HandlerAdapter.

Issue: SPR-15250

Currently the BOM versions are:

* reactor-core 3.0.6.BUILD-SNAPSHOT
* reactor-netty 0.6.2.BUILD-SNAPSHOT

This commit fixes as well a few deprecations in reactor-core.

The WebTestClient API no longer provides access to a base
ExchangeResult without a decoded response body.

Instead the response has to be decoded first and tests can then
access the EntityExchangeResult and FluxExchangeResult sub-types.

This commit adds an overloaded DataBufferUtils.read method that operates
on a AsynchronousFileChannel (as opposed to a ReadableByteChannel, which
already existed). This commit also uses said method in the Resource
encoders, if the Resource is a file.

The SSE reader is ordered ahead of StringDecoder because with
response.decodeToFlux(String.class) we actually want the SSE reader
to get involved first based on the content-type.

At the same time with response.decodeToMono(String.class) there is
nothing the SSE reader can do while the StringDecoder could read the
entore content as one String, as long as the server does terminate
the stream which can happen in a testing scenario.

This commit updates ServerSentEventHttpMessageReader#readMono in
to support String.class by delegating to StringDecoder. Since
reading to a Mono is an explicit choice there is not much possibility
for interfering with decoding to Flux.

Issue: SPR-15331

This commit simplifies the use of DataBufferFactory in the SSE reader
which is used only to wrap a byte[] as a DataBuffer. There is no actual
benefit to use anything other than a DefaultDataBufferFactory.

Add default readers and writers from separate methods instead of
having them interleaved which is hard to read and easy to miss.

Issue: SPR-15331

Issue: SPR-15324