Prior to this commit, the resource handling chain and its
`ResourceResolvers` would use specific `Resource` implementations in
order to add resource metadata to the HTTP response. For example,
`VersionedResource` and `EncodedResource` are both adding specific HTTP
response headers.

This commit aims at making this mechanism more stable and reusable,
since the previous implementation would fail in case a resolved resource
would be both a `VersionedResource` wrapping a `EncodedResource` (or the
other way arount). Only one of the specific implementations would
contribute its metadata since the code supporting that in
`ResourceHttpRequestHandler` would only check for `instanceof` tests,
whereas those implementations are acutally delegating calls to
the wrapped resource.

Now both `VersionedResource` and `EncodedResource` have been replaced by
specific implementations of `ResolvedResource`, which directly provides
those HTTP response headers as part of `getResponseHeaders()`.

This commit applies the same changes for the web reactive
implementations and its `ResourceWebHandler`.

Issue: SPR-14264

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

Issue: SPR-14484

This change fixes a NullPointerException in GzipResourceResolver, which
assumed that calls to the `resolveResource` method were made with only
non-null values for request.

This is not the case for the VersionResourceResolver, which tries to
resolve resources that aren't requested per se by the HTTP request.

Issue: SPR-13149

Also respect HandlerMapping order in ResourceUrlProvider

- ResourceResolver and ResourceResolverChain now have a consistent API
  with regard to method names and terminology.

- ResourceResolver and ResourceResolverChain now accept
  List<? extends Resource> instead of List<Resource> for simplified
  programmatic use.

- Improved Javadoc across the package.

- Formatted code to align with standards.

- Removed all references to ResourceUrlPathTranslator.

Issue: SPR-10933

An initial commit with expanded support for static resource handling:

- Add ResourceResolver strategy for resolving a request to a Resource
  along with a few implementations.

- Add PublicResourceUrlProvider to get URLs for client-side use.

- Add ResourceUrlEncodingFilter and
  PublicResourceUrlProviderExposingInterceptor along with initial
  MVC Java config support.

Issue: SPR-10933

This reverts commit d4a0e628, reversing
changes made to 8abe9497.

This change splits out resource transformation out from the
ResourceResolverChain so that chain is focused entirely on resource
resolution (as its name suggests). The invocation of transformers
is left as a separate step, it uses a different (recursive)
algorithm in any case and iterates over a different set of objects.

Also ResourceResolverChain is now limited strictly to methods that
a ResourceResolver should be able to use to delegate to remaining
resolvers.

Furthermore, ResourceResolverChain now maintains an internal index
of the "current" resolver so that resolvers don't have to pass the
chain when invoking it much like a (Servlet API) FilterChain works.
If the last resolver calls the chain again, a null value is returned.