This commit creates auto-configuration classes for both the
annotation and functional variants of the WebFlux framework.

They provide the basic support to get started with those, by
creating the required `HttpHandler` using the provided application
context (for annotation) or `RouterFunction`s (for functional).

They do support `WebFilter` registration and a few advanced
features such as resource handling, `messageReaders|Writers`
and `ViewResolver` auto-registration.

Closes gh-8386