An `HttpHandler` bean must be provided once the infrastructure triggered
by `@EnableWebFlux` has been processed. Rather than creating a
`HttpHandler` in that auto-configuration, this commit moves it to a
dedicated auto-config, like we do for `DispatcherServlet` for
servlet-based webapps.
As this is the only bean we auto-configure in a functional fashion, the
`WebFluxFunctionalAutoConfiguration` is now merged with this new
auto-configuration, making its purposes clearer.
Cloess gh-8436
deaa6089b0