While View and ViewResolver play the same role as in spring-webmvc they
are now abstracted behind the HandlerResultHandler abstraction so that
top-level contracts don't reference them and the DispatcherHandler is
also unaware of their existence.

Furthermore view resolution and response body handling which are now at
the same level of abstraction (each is a HandlerResultHandler) will
also share code for content negotiation, so it makes sense for them to
be side by side.

This commit moves the reactive.view package to reactive.result.view
with the View and ViewResolver contracts (previously in the top-level
reactive package) also moving there.

 - @PathVariable
 - @RequestHeader
 - @RequestParam
 - @CookieValue
 - @Value
 - @RequestAttribute
 - @SessionAttribute

update after to then (WIP flux)

Rename two classes each adapting to WebHandler to avoid confusing them:
1. HttpWebHandlerAdapter adapts from the low level HttpHandler to any
WebHandler (e.g. DispatcherHandler).
2. SimpleHandlerAdapter adapts the plain WebHandler for use within the
DispatcherHandler.

This commit also fixes an issue in WebHttpHandlerBuilder to ensure that
WebExceptionHandler's are inserted before and not after WebFilter's.

This commit replaces the temporary SimpleUrlHandlerMapping with
a full-featured one.

The renaming makes it clear this exception is for use where 400 error
applies within a Spring web application where the error may be
associated with a MethodParameter, a BindingResult, and so on.

There is no need for BadRequestStatusException which can be expressed
with ResponseStatusException(HttpStatus.BAD_REQUEST, "reason").

This commit wraps up the previous commits:

 - It uses HttpMessageConverter in the web.reactive.server package instead of Encoder/Decoder.
 - It introduces tests for the Resource @ResponseBodies.

This commit replaces use of the existing ServletException-based
exceptions with the new ones from ~.web.server.

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

Rename to MappingContentTypeResolver and replace "fileExtension" with
"key" in methods to be more general (e.g. file extension or query
param). Also switch from List to Set for the return values.

This commit adds RequestMappingInfoHandlerMapping and
RequestMappingHandlerMapping with support equivalent to that in
spring-webmvc.

A pretty complete equivalent to the same in spring-webmvc except for
CORS checks, and custom HTTP methods. Another notable difference is
that the "params" condition works on query params strictly.

This commit adds AbstractHandlerMethodMapping, a starting point for
AbstractHandlerMapping, and HttpRequestPathHelper with a similar
purpose to UrlPathHelper but based with ServerWebExchange as input.

The new package is parallel to the "mvc" package under spring-webmvc
and a place to hold the various programming model styles.