GH-3183: Add ReactiveRequestHandlerAdvice

[artembilan]

Fixes #3183

• Introduce a ReactiveRequestHandlerAdvice with a BiFunction<Message<?>, Mono<?>, Publisher<?>>
  logic to apply a Mono.transform() operator for a returned from the handler Mono reply
• Fix WebFluxRequestExecutingMessageHandler to return a Mono.then() instead of an explicit subscription -
  it happens downstream anyway during reply producing with a proper error handling, too
• Demonstrate ReactiveRequestHandlerAdvice in the RSocketDslTests - without retry() it fails
• Add ConsumerEndpointSpec.customizeMonoReply() for convenience
• Document ReactiveRequestHandlerAdvice feature

[artembilan]

If the solution is good, I'll go ahead with WebFlux reworking.

This feature opens for us a possible to apply a TransactionInterceptor with a ReactiveTransactionManager onto our reactive-replying components.

I may admit that this could be kinda breaking change though since that modified RSocketOutboundGateway is not an AbstractReplyProducingMessageHandler anymore...

[sirimamilla]

HI @artembilan,

This will add a lot of value. As it gives a lot of scope for customization and unleashes the power of Reactive framework.

[garyrussell]

If the solution is good, I'll go ahead with WebFlux reworking.

LGTM

[artembilan]

I'll come back to this after merging #3201 .
They are kinda depend and complement each other.

[artembilan]

I have changed my mind: now ReactiveRequestHandlerAdvice checks a type of the return from the invocation.proceed().
So, we don't need extra abstraction to enforce reactive behavior for target MessageHandler.