Issue: SPR-14557

Issue: SPR-14735

Refactor BodyInsertor to expose insertion logic directly, rather than
exposing a writer function and supplier.

This commit refactors the response body extraction logic into a separate
function: BodyExtractor. Standard populators can be found in
BodyExtractors.

This commit extracts the response body insertion logic into a separate
strategy interface: BodyPopulator. Standard populators can be found in
BodyPopulators.

This commit refactors the Router into a RoutingFunctions class, by:

  - Renaming the class :)
  - Moving all Configuration logic into a separate, top-level
  Configuration class with mutable builder.

Issue: SPR-14693

Issue: SPR-14521

Removed accidentally added ModelAndView class.

In RoutingFunction, renamed 'and' to 'andSame', and 'andOther' to
'and' to make the commonly used method name shorter.

Merged all *Response implementations into one DefaultResponses class

This commit introduces template rendering support in the web.reactive
package, through a Response.render method and a Rendering interface.

Issue: SPR-14557

Issue: SPR-14557

This commit changes the default file extension configured with
`AppCacheManifestTranformer`. This ResourceTransformer was previously
considering `.manifest` files by default, but this has been changed in
the official spec to `appcache`, in order not to clash with Microsoft's
unregistered application/manifest type.

Issue: SPR-14687

`ResolvedResource` is a rather generic name - changing the extended
interface to something more meaningful: `HttpResource`.
For now, implementations are linked with the resource handling chain,
but this aspect has been removed from the interface documentation.

Issue: SPR-14264

Prior to this commit, FreeMarkerView used the system default charset to
render. This commit switches this by defaulting to UTF-8, if no charset
is specified in the content type.

 - Add contentType parameter to AbstractView.renderInternal, used to
 determine the charset contained therein
 - Adds a defaultCharset property to AbstractView and
 ViewResolverSupport.

Changed View's render method from taking a HandlerResult to taking a
Map<String, ?>, in order to facilitate scenarios where a HandlerResult
is not available (i.e. web.reactive.function).

 - Changed Response.sse methods to allow for specific Publisher types to
 be returned in Response.

Two changes to web.reactive.function:

 - Changed Response.stream method to allow for specific Publisher types
 to be returned in Response.

 - Router now stores HttpMessageReader|Writer retrieved from
 Configuration in the attributes as supplier, not as stream, to allow
 for multiple reads.

Removes attributes() method from Request, as it exposes a mutable map.

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

Issue: SPR-14521

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

This commit fixes `ResponseEntityResultHandler` so that it only tries to
call `writeBody` if the `ResponseEntity` is not null. In case the
response entity body is null, the response is flushed right away and the
request is signaled as handled.

Issue: SPR-14663

This change puts the MockHttpServerRequest/Response under
org.springframework.mock.http.server.reactive.test which
mirrors the arrangement we have with the Servlet mock equivalents
and sets up the addition of MockHttpServerRequest/Response in
spring-test.

Issue: SPR-14421

Issue: SPR-14421

Issue: SPR-14522

Issue: SPR-14522

Issue: SPR-14658