!str #15742 Hook-up groupBy to new DSL

* Produces Flows instead of Publisher
!str #15742 Hook-up prefixAndTail to new DSL

* Produces tail with flows instead of Publisher
!str #15742 Hook-up splitWhen to new DSL

* Produces flows instead of publishers
!str #15742 Hook-up flatten (concat all) to new DSL

* input streams of flows, instead of publisher
Merge pull request #15824 from akka/wip-15742-flow-of-flows-patriknw

!str #15742 Hook-up flow of flows to new DSL
=str #15739 Hook-up grouped, groupedWithin, timerTransform to new DSL
=str #15738 Hook-up drop, dropWithin, take, takeWithin to new DSL
=str #15741 Hook-up conflate, expand, buffer, and FoldSink