I edited for the usual stuff: spelling, punctuation,
grammar, formatting, usage, and voice.

Before this commit, Kotlin inner class nested configuration
handling thrown an IndexOutOfBoundsException due to bogus filtering
of its constructor parameter reference to an instance of the outer
class.

This commit keep constructor parameter of type INSTANCE in order to
throw a more meaningful NoSuchBeanDefinitionException.

Issue: SPR-17222

1. Rename globalResources to useGlobalResources.
2. Use of global resources is mutually exlusive with explicit config.
3. Allow Consumer<HttpResources> to configure global resources.
4. Allow ConnectionProvider + LoopResources Supplier to customize
   creation and initialization.
5. Do not manage externally provided ConnectionProvider + LoopResources
   instances.

Issue: SPR-17243

I edited for spelling, punctuation, grammar, usage, and corporate voice. I also added cross-references and links to the Javadoc in a number of places.

Closes gh-1934

JettyResourceFactory, similar to ReactorResourceFactory, allows
to share resources (Executor, ByteBufferPool, Scheduler) between
Jetty clients and servers.

Issue: SPR-17179

Issue: SPR-17189

This reverts commit 7e917335.

This commit handles "empty" cases for `ResponseEntity` controller
handler return types when wrapped with a `java.util.Optional` in Spring
MVC or a single `Publisher` like `Mono`.

Given the following example for Spring MVC:

```
@GetMapping("/user")
public Optional<ResponseEntity<User>> fetchUser() {
	Optional<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

If the resulting `Optional` is empty, Spring MVC will infer a
`ResponseEntity` with an empty body and a 404 HTTP response status.

The same reasoning is applied to Spring WebFlux with Publisher types:

```
@GetMapping("/user")
public Mono<ResponseEntity<User>> fetchUser() {
	Mono<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

This feature is only valid for `HttpEntity` return types and does not
apply to `@ResponseBody` controller handlers.

Issue: SPR-13281

Issue: SPR-17180
Issue: SPR-17182

Rename "Builder" sub-section to "Configuration" and move it in the
beginning before all others since it explains how to create a client
in the first place.

Update content on Reactor Netty connector based on the API in 0.8 and
specifically address Reactor Netty resources and lifecycle.

Issue: SPR-16963

Issue: SPR-17167

Issue: SPR-16946

Issue: SPR-17145

Issue: SPR-17143

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-17104

Issue: SPR-17032

Issue: SPR-16993

Obvious Fix

Issue: SPR-16993

Issue: SPR-17095

This commit introduces Protobuf support in WebFlux via dedicated
codecs.

Flux<Message> are serialized/deserialized using delimited Protobuf
messages with the size of each message specified before the message
itself. In that case, a "delimited=true" parameter is added to the
content type.

Mono<Message> are expected to use regular Protobuf message
format (without the size prepended before the message).

Related HttpMessageReader/Writer are automatically registered when the
"com.google.protobuf:protobuf-java" library is detected in the classpath,
and can be customized easily if needed via CodecConfigurer, for example
to specify protocol extensions via the ExtensionRegistry based
constructors.

Both "application/x-protobuf" and "application/octet-stream" mime types
are supported.

Issue: SPR-15776

1. Revise @SubscribeMapping to address common points of confusion.
2. Add ExecutorSubsribableChannel.
3. Split Events and Interception in two.

Issue: SPR-16950

Issue: SPR-13989

After the latest changes, two small fixes in the clone method to copy
the encode flag, and in the encodeUriTemplate method to account for
possible null query params.

Improvements in the URI encoding section.

Issue: SPR-17039, SPR-17027

(cherry picked from commit 4341838a)

Leverage https://github.com/jetty-project/jetty-reactive-httpclient
to add support for Jetty in WebClient via JettyClientHttpConnector.

Implemented with buffer copy instead of optimized buffer wrapping
because the latter hangs since Callback#succeeded doesn't allow
releasing the buffer and requesting more data at different times
(required for Mono<DataBuffer> for example).
See https://github.com/eclipse/jetty.project/issues/2429.

Issue: SPR-15092

Closes gh-1887