`FluxMessageChannel`: propagate the current context from producer to consumer

[artembilan]

There is no connection between producer (FluxMessageChannel.send()) and consumer FluxMessageChannel.subscribe(Subscriber) to propagate the context (typically thread local variables), e.g. an Observation or an Authentication.
Therefore we need to look into mechanism to transport the context via IntegrationMessageHeaderAccessor.REACTOR_CONTEXT message header.