Forwarded headers are now processed before ServerWebExchange is created
through ForwardedHeaderTransformer which has the same logic as the
ForwardedHeaderFilter but works on the request only.

ForwardedHeaderFilter is deprecated as of 5.1 but if registered it is
removed from the list of filters and ForwardedHeaderTransformer is used
instead.

Issue: SPR-17072

Issue: SPR-17094

Update all classes so that inner classes are always last. Also
ensure that utility classes are always final and have a private
constructor and make exceptions final whenever possible.

Issue: SPR-16968

Issue: SPR-16972

Issue: SPR-16898

Issue: SPR-16298

Issue: SPR-15720
Issue: SPR-15792

This commits extends nullability declarations to the field level, formalizing the interaction between methods and their underlying fields and therefore avoiding any nullability mismatch.

Issue: SPR-15720

Issue: SPR-15674

Replace the more limited List-based methods to add filtes and exception
handlers with Consumer<List<?>> variants.

This commit introduces LocaleContextResolver interface, which is used
at ServerWebExchange level to resolve Locale, TimeZone and other i18n
related informations.

It follows Spring MVC locale resolution patterns with a few differences:
 - Only LocaleContextResolver is supported since LocaleResolver is less
   flexible
 - Support is implemented in the org.springframework.web.server.i18n
   package of spring-web module rather than in spring-webflux in order
   to be able to leverage it at ServerWebExchange level

2 implementations are provided:
 - FixedLocaleContextResolver
 - AcceptHeaderLocaleContextResolver

It can be configured with both functional or annotation-based APIs.

Issue: SPR-15036

Issue: SPR-15501

WebHttpHandlerBuilder is low level not expected to appear frequently
but they might be repeated more in tests (e.g. of WebFilter's).

Issuse: SPR-15499

With this commit, ServerCodecConfigurer is now exposed as a bean in
order to be provided to DefaultServerWebExchange via
WebHttpHandlerBuilder and HttpWebHandlerAdapter. This allows
DefaultServerWebExchange to get configured codecs for reading form or
multipart requests.

Issue: SPR-14546

CompositeHttpHandler is public and called ContextPathCompositeHandler.

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

Issue: SPR-15074

MockServerHttpRequest and MockServerHttpResponse now extend the same
abstract base classes that server-specific implementations do and
therefore approximate their behavior more closely.

As an immediate consequence MockServerHttpRequest is read-only after
it is created. Instead it now exposes static builder methods similar
to those found in RequestEntity. This enforces more strictness as well
as recycling of requests in tests and provides nicer builder methods.

To simplify tests DefaultServerWebExchange now offers a constructor
with just a request and response, and automatically creating a
DefaultWebSessionManager.

The spring-test module now also contains client-side reactive mock
request and response implementations. The mock client request extends
the same AbstractClientHttpRequest as client-specific implementations
do. There is no abstract base class for client responses.

Issue: SPR-14590

This commit adds support for detecting the target WebHandler along with
WebFilters, WebExceptionHandlers, and other spring-web reactive
strategies in an ApplicationContext.

WebReactiveConfigurationSupport has @Bean factory methods for
DispatcherHandler and ResponseStatusExceptionHandler.

WebHttpHandlerBuilder has a static factory method that initializes the
builder from an ApplicationContext. This method is also used in the
DispatcherHandler#toHttpHandler(ApplicationContext) shortcut method.

Issue: SPR-14837

Rename two classes each adapting to WebHandler to avoid confusing them:
1. HttpWebHandlerAdapter adapts from the low level HttpHandler to any
WebHandler (e.g. DispatcherHandler).
2. SimpleHandlerAdapter adapts the plain WebHandler for use within the
DispatcherHandler.

This commit also fixes an issue in WebHttpHandlerBuilder to ensure that
WebExceptionHandler's are inserted before and not after WebFilter's.

WebServerExchange -> ServerWebExchange

Follows the same convention as in the http package also better allowing
the possibility for a client equivalent in the future.

WebToHttpHandlerBuilder -> WebHttpHandlerBuilder
WebToHttpHandlerAdapter -> WebHttpHandlerAdapter

More consistent with Spring conventions.