Consider Protobuf codecs auto-configuration for RSocket support

[bclozel]

As of #16021, Spring Boot supports RSocket as a server. Protobuf is a popular solution for messages (de)serialization and it seems to align well with the RSocket paradigm.

This issue should consider whether and how we should:

• auto-configure Protobuf codecs for RSocket
• provide configuration properties
• provide dependency management
• do additional work in the Spring Boot application builds

[eiswind]

As of 2.2.2 I had to disable RSocketServerAutoConfiguration and add:

    @Bean
    CloseableChannel server(GreetingServiceServer serviceServer){
        return RSocketFactory
                .receive()
                .acceptor((setup, sendingSocket) -> Mono.just(
                        new RequestHandlingRSocket(serviceServer)
                ))
                .transport(TcpServerTransport.create(serverPort))
                .start()
                .block();
    }

I did not manage to use a protobuf service and a spring messagehandler in the same application.

Perhaps it is possible to autodetect the rpc-services?