Issue: SPR-15770

Issue: SPR-15769

Issue: SPR-15720

Issue: SPR-15758

This commit introduces overloaded variants of `bodytoMono`,
`bodyToFlux`, `toEntity`, and `toEntityList` that take a
`ParameterizedTypeReference`. It also adds similar methods to
`WebClient.ResponseSpec`.

Issue: SPR-15725

Issue: SPR-15720

This binary format more efficient than JSON should be useful for server
to server communication, for example in micro-services use cases.

Issue: SPR-15424

Issue: SPR-15757

Issue: SPR-15766
(cherry picked from commit c9ffa641)

Issue: SPR-15763
(cherry picked from commit aa0d7a61)

Issue: SPR-15758

Issue: SPR-15763

Issue: SPR-15760

Avoid re-using the WriteBarrier as the Subscription to both the
completionSubscriber and the writeSubscriber.

Instead DownstreamBridge is now called WriteCompletionBarrier and is
both a Subscriber and Subscription, i.e. handles all signals to and
from the completion Subscriber.

This frees the request method implementation in WriteBarrier to assume
it can only be called by the writeSubscriber and hence it's safe to
pass on request signals to the write source outside the synchronized
block.

This commit introduces a boolean `readOnly` flag in `HttpHeaders`,
which is used to check if creating a new `HttpHeaders` instance is
necessary in `readOnlyHttpHeaders`.

* pr/1469:
  Polish contribution
  Use dedicated addAdvisors

Closes gh-1469

See gh-1469

* pr/1465:
  Fix typo

closes gh-1465

As a follow-up of the ApplicationContext Kotlin extensions, close to
the Kotlin functional WebFlux DSL and partially inspired of the
Groovy/Scala bean configuration DSL, this commit introduces a
lightweight Kotlin DSL for functional bean declaration.

It allows declaring beans as following:

beans {
	bean<Foo>()
	profile("bar") {
		bean<Bar>("bar", scope = Scope.PROTOTYPE)
	}
	environment({ it.activeProfiles.contains("baz") }) {
		bean { Baz(it.ref()) }
		bean { Baz(it.ref("bar")) }
	}
}

Advantages compared to Regular ApplicationContext API are:
 - No exposure of low-level ApplicationContext API
 - Focused DSL easier to read, but also easier to write with a fewer
   entries in the auto-complete
 - Declarative syntax instead of functions with verbs like registerBeans
   while still allowing programmatic registration of beans if needed
 - Such DSL is idiomatic in Kotlin
 - No need to have an ApplicationContext instance to write how you
   register your beans since beans { } DSL is conceptually a
   Consumer<GenericApplicationContext>

This DSL effectively replaces ApplicationContext Kotlin extensions as
the recommended way to register beans in a functional way with Kotlin.

Issue: SPR-15755

This commit introduces an apply method to `WebClient.Builder`, allowing
users to make multiple changes to the builder in one consumer.

Issue: SPR-15743

The failures look like older failures possibly exposed by recent
changes in Reactor.

The one in ViewResolutionResultHandler is very old test error.
The one in Jackson2JsonDecoderTests is more recent but went unreported.

WriteBarrier acts as Subscription to both completionSubscriber and
writeSubscriber so the request method needs to be careful not to
pass on the request outside of readyToWrite or writeSubscriber checks.

1. Group by contract
2. Follow lifecycle -- Subscriber 1st, Subscription 2nd, Publisher last
3. Order of declaration in implemented contracts

Add support for CoreSubscriber context passing

ContentNegotiationManagerFactoryBean now provides an option to
explicitly set the strategies to use vs customizing a fixed
list of default strategies.

Issue: SPR-11114

Issue: SPR-15649

This commit moves `toEntity(Class<T>)` and `toEntityList(Class<T>)`
from WebClient.ResponseSpec to ClientResponse. The main reason for doing
so is that the newly introduced `onStatus` method (see
2f9bd6e0) does not apply to these two
methods, and the result would be confusing. Also, `ClientResponse` and
`ResponseEntity` represent the same data: status code, headers, and a
body.

Issue: SPR-15724

This commit introduces an ExchangeFilterFunction that throws an
exception given a HTTP status predicate.

Issue: SPR-15724

This commit introduces a way to customize the WebClientExceptions, as
thrown by WebClient.ResponseSpec.bodyTo[Mono|Flux]. The first
customization will override the defaults, additional customizations are
simply tried in order.

Issue: SPR-15724

This commit adds the attribute(String) convenience method to the
ClientRequest class. It also adapts the Basic Authentication filter to
use it.

This commit changes `ServerRequest.attribute(String)`` to return
`Optional<Object>` instead of `Optional<T>`, where `T` was infered
from a type parameter.