Related to: https://github.com/spring-projects/spring-boot/issues/18812
* Extract `ServerRSocketMessageHandler` into a `public` class to allow
to configure it as top-level bean and bind it into an existing server
* Change `ServerRSocketConnector` to accept the mentioned external bean
and don't create an internal RSocket server with an assumption that it
is create externally
* Add `IntegrationRSocketMessageHandler.requestMappingCompatible`
option to allow to configure `ServerRSocketMessageHandler` for both
Spring Integration RSocket channel adapters and regular `@MessageMapping`.
This is useful for Spring Boot auto-configuration
These changes give a hook to auto-configure Spring Integration RSocket
channel adapters in Spring Boot
04ff879f7e