Issue: SPR-14557

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.

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

 - 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.

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

This commit introduces a new, functional web programming model in the
org.springframework.web.reactive.function package. The key types
are:

 - Request and Response are new Java 8-DSLs for access to the HTTP
   request and response
 - HandlerFunction represents a function to handle a request to a
   response
 - RoutingFunction maps a request to a HandlerFunction
 - FilterFunction filters a routing as defined by a RoutingFunction
 - RequestPredicate is used by Router to create RoutingFunctions
 - RequestPredicates offers common RequestPredicate instances