Extract WebClient content into a separate file that is now included
both in the WebFlux and in the Integration sections.

This allows having RestTemplate and WebClient documented in one place
under Integration while also keeping the same included in the WebFlux
section too.

Issue: SPR-15998

Prior to this commit, asking for a `Void` type using any of the
`ClientResponse#bodyTo*` methods would immediately return an empty
`Publisher` without consuming the response body.

Not doing so can lead to HTTP connection pool inconsistencies and/or
memory leaks, since:

* a connection that still has a response body being written to it cannot
be properly recycled in the connection pool
* incoming `DataBuffer` might not be released

This commit detects when `Void` types are asked as body types and in
those cases does the following:

1. Subscribe to the response body `Publisher` to allow the connection to
be returned to the connection pool
2. `cancel()` the body `Publisher` if the response body is not empty; in
that case, we choose to close the connection vs. consume the whole
response body

Those changes imply that `ClientHttpResponse` and other related
contracts don't need a `close()` method anymore.

Issue: SPR-16018

Includes unified detection of Kotlin's optional parameters in MethodParameter.isOptional(), reduces BeanUtils.findPrimaryConstructor to Kotlin semantics (for reuse in AutowiredAnnotationBeanPostProcessor), and finally introduces a common KotlinDetector delegate with an isKotlinType(Class) check.

Issue: SPR-15877
Issue: SPR-16020

Issue: SPR-14524

Issue: SPR-14524

Issue: SPR-15994

Issue: SPR-14524

Issue: SPR-16017

(cherry picked from commit 40ba95f8)

Issue: SPR-14524

Issue: SPR-15656

Prior to this issue, SPR-15920 added this new `close()` method which was
supposed to be called to clean resources after response processing.

This commit changes the meaning of that method: calling `close()` will
close the underlying HTTP connection. This has to be called if the
response body is not consumed by the application, since at that point
the underlying connection might be in an inconsistent state if shared in
a connection pool.

Issue: SPR-15993

This commit add the default constructor if available as
fallback after to the primary constructor.

Issue: SPR-16012

Issue: SPR-15963

Issue: SPR-15960

Now that WebSessionStore is in charge of expiration checks on retrieve
it makes sense to also update the lastAccessTime on retrieve at the
same time, saving the need to call it after a retrieve.

Issue: SPR-15963

Issue: SPR-15963

Issue: SPR-16014

Issue: SPR-15974

Issue: SPR-16014

Issue: SPR-16009

Pending -- WebSocket, WebTestClient, more details around annotation
processing, exception handling, and view resolution.

Issue: SPR-15149, SPR-16009

Issue: SPR-15934

It is no longer adequate to list supported arguments and return values
-- between Web MVC and WebFlux, directly on the annotation.

Instead we have tables in the respective chapters in the reference with
cross references to each other.

Issue: SPR-15149

`jackson-datatype-jdk7` is not provided anymore. It's handlers are
incorporated in core databind.

Issue: SPR-15778

Issue: SPR-15989