Improve RSocketRequester.ResponseSpec Kotlin extensions

[juergenzimmermann]

Affects: Spring Framework 5.2.0-M3

It would be nice to have Kotlin extension functions for RSocketRequester.ResponseSpec so that one could write in Kotlin style, e.g.:

requester.route("find-by-id")
    .data(CustomerIdRequest(id))
    .retrieveMono<Customer>()

instead of

requester.route("find-by-id")
    .data(CustomerIdRequest(id))
    .retrieveMono(Customer::class.java)

In the same way it would be nice to have .retrieveFlux<T>()

[sdeleuze]

Another improvement I will make is to leverage d6b5c20 to replace dataFlow(flow: Flow) extension by a general purpose reified data(producer: Any) one.