Make it easy to run both Spring Messaging and RSocket-RPC simultaneously

[bsideup]

As a user of rsocket-rpc-java in Liiklus, I would like to be able to reuse Spring {Framework,Boot}'s RSocket support, and be able to handle rsocket-rpc requests while having some routes (as per RSocket's Routing) defined as Spring Messaging's handlers.

Expectation:

1. I create an instance of RSocketRPCRouteHandler that rsocket-rpc generated for me
2. I register it to some registry of RouteHandlers
3. While handling RSocket requests, Spring's RSocket support delegates to some abstraction that uses registered RouteHandlers to decide which one should handle the request

This all is very similar to the HTTP support & Servlets API, where a Servlet can be added next to Spring Framework's DispatcherServlet and handle a subset of requests.

[rstoyanchev]

This requires a lower level routing mechanism, so I've created rsocket/rsocket-java#974. When that mechanism becomes available, I expect it to be possible to plug multiple, independent RSocket handlers which by definition should be independent of each other and not be aware of each other. So I'll close this for now and we can create a new one later when it's more obvious what if anything needs to be done here or in Spring Boot more likely where the server is bootstrapped and configured.