In the case of a WebFlux + RSocket over websocket setup, the RSocket
auto-configuration would not set up the required routes; only the
websocket endpoint for RSocket would be available, overriding the
handler configured for WebFlux.
This commit introduces `NettyRouteProvider`. Components implementing
that interface can contribute HTTP routes to the Reactor Netty server
being built.
* if none is provided, the regular handler setup is used
* if one or more routes are provided, routes are sorted and added before
the WebFlux handler (acting as a default)
Fixes gh-16826
0b70862627