Add Java DSL .fluxTransform(Function) operator

[artembilan]

For better interoperability with Reactor Flux from the
end-user perspective introduce an operator which could
call a target Function with integration data wrapped to the Flux
and expect a Publisher<?> result to continue the flow.
This way end-user just needs to implement a Function (or method)
to accept the Flux as an input and return some Publisher
after the sequence of reactive operators.

Such a new operator also allows a smooth integration with the
Spring Cloud Function, where it is just enough to lookup the function
in the catalog and inject it into this operator

[artembilan]

@olegz ,

This is what we talked about how to avoid extra call for the source.output() when we mix Spring Integration Java DSL and Spring Cloud Function.
Also pay attention how I use Mono.subscriberContext() to propagate a requestMessage to be used for building reply message.

Let me know WDYT!

Thank you

/CC @smaldini

[garyrussell]

Please update the commit comment with rationale.

[artembilan]

Pushed.
Thank you for the pointer!

[artembilan]

Do not merge yet: I'm going to move the body of that fluxTransform() to the separate class.
Too much noise in so huge already IntegrationFlowDefinition...

[olegz]

FWIW, I know Gary already merged it, but as Artem said this came up as he was reviewing one of the PRs for SCSt/SCF integration work, so I can definitely use it now.