Issue: SPR-15367

This commit introduces a `useRegisteredExtensionsOnly` property that
indicates whether classes that use the `MediaTypeFactory` for supplying
default media types can do so.

 - In classes that were introduced in Spring 5.0, the
 `useRegisteredExtensionsOnly` property takes the place of the
 `useJaf` property that was removed in 0aaa6528
 - In classes that existed before Spring 5.0, the
 `useRegisteredExtensionsOnly` property is added in addition to the
 deprecated `useJaf`, the latter delegating to the former, but with
 flipped behavior.

Issue: SPR-14908

This commit updates the main code base to conform to the dropped JAF
dependency in MediaTypeFactory. Specifically, it

 - Removes JAF detection (JAF_PRESENT constants)
 - Deprecated useJaf properties, with no direct replacement.
 - Updated docs to remove JAF references, in favor of MediaTypeFactory.

Issue: SPR-14908

Issue: SPR-15190

A straight-forward port of the resource handling support in
spring-webmvc to spring-web-reactive. Primarily adapting contracts and
implementations to use the reactive request and response and the
reactive ResourceHttpMessageWriter.

Issue: SPR-14521

Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

Now that we have a CompositeContentTypeResolverBuilder with built-in
defaults, we switch to those defaults in places where a
ContentTypeResolver is used.

This is the equivalent of the existing ContentNegotiationManager +
ContentNegotiationManagerFactoryBean