Issue: SPR-14879

Prior to this commit, the wrong `HandlerResultHandler` could be
resolved when handling exceptions; this could happen only if the
original handler and exception handler had different signatures:

```
Publisher<String> originalHandler() { ... }

@ExceptionHandler(MyCustomException.class)
ResponseEntity<Mono<String>> handleException() { ... }
```

In that case, the `ResponseBodyResultHandler` would be used when
handling exceptions instead of the `ResponseEntityResultHandler`.

This commit ensures that the `HandlerResult` returned by the exception
handler is used to resolve the `HandlerResultHandler`. The latter will
process the result and use it to write to the HTTP response.

Issue: SPR-14876

Issue: SPR-14867

Issue: SPR-10502

Issue: SPR-14583

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 commits adds a validation check whether the SockJS session type
matches the transport type and rejects requests for which they
don't match.

Issue: SPR-14867

Issue: SPR-14800

Issue: SPR-14872
(cherry picked from commit 17863dfb)

Issue: SPR-14863

Issue: SPR-14863

Issue: SPR-14869

 - ScriptedSubscriber has been renamed to Verifier
 - The Publisher is passed to create() instead of verify()
 - No more need to specify the generic type explicitly
 - Version is now sync with reactor-core

Issue: SPR-14800

For SPR-14863 we need to adjust the code generation for OpNE
to use !x.equals(y) rather than x!=y. There are also further
cases in the equalityCheck() code in Operator that were not
being handled in the compilation case (when comparators are
used for example). This latter issue also affects OpEQ.

Rather than add yet more bytecode generation, both OpNE and
OpEQ generateCode() methods have been simplified. The
generated code now delegates to equalityCheck() in Operator
which is exactly what the interpreted case does.

This ensures that the compiled code continues to behave just
like the interpreted case. It ensures changes to the interpreted
case are automatically picked up for the compiled case. It
makes the bytecode generation simpler.

The benefit of compilation of SpEL expressions is to avoid
slow reflective calls - that doesn't apply for a basic
(in)equality test so there is no need to go crazy in bytecode
gen.

Issue: SPR-14863

The method to access the Principal from the ServerWebExchange is now
a Mono<Principal> (rather than Optional<Principal>).

There is also support for Principal as a controller method argument.

Issue: SPR-14680, SPR-14865

(cherry picked from commit 77e00f19)

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

Issue: SPR-14543

The createBinder method in BindingContext now returns the binder
instance rather than a Mono with the expectation that binder
initialization (e.g. @InitBinder) does not require blocking.

Issue: SPR-14543

HandlerMethodArgumentResolver is a non-blocking contract, however only
implementations that read the request body require blocking.

This commit introduces SyncMethodArgumentResolver as an extension of
the non-blocking contract that allows synchronous implementations to
use synchronous argument resolution.

There is also a SyncInvocableHandlerMethod extension that uses only
sync argument resolvers and allows a synchronous invocation.

Issue: SPR-14543

Issue: SPR-14862
(cherry picked from commit d859826b)

Issue: SPR-14304

Issue: SPR-14861

This commit introduces the `DispatcherServletCustomizer` callback
interface that can be used to customize the `DispatcherServlet` that a
`MockMvc` is using.

Previously, only the `dispatchOptions` flag can be customized. This
commit allows to customize any property of `DispatcherServlet` before it
is initialized.

Issue: SPR-14277

Includes consistent quoting of qualified type names in related classes.

Issue: SPR-14831

Issue: SPR-14816

Issue: SPR-14783