Support resolution of AuthenticatedPrincipal in RSocket handler methods

[joshlong]

It would be mighty convenient to be able to inject the current authenticated user in an RSocket controller handler method

@MessageMapping ("test") 
Mono<String> hello (@Authenticated Principal p){ 
  return Mono.just ("Hello, "  + p.getName()) ;
} 

Per a discussion with @rwinch it looks like the autoconfiguration needs an RSocketMessageHandlerCustomizer that adds AuthenticationPrincipalArgumentResolver in RSocketSecurityAutoConfiguration.java

[philwebb]

Closing in favor of PR #28927. Thanks @terminux!