!str #15236 Replace Transformer with Stage

* replace all existing Transformer with Stage (PushPullStage)
* use Flow[ByteString, ByteString] as encoder/decoder transformer in http
* use the IteratorInterpreter for strict if possible
* emit then become
* emit then finish
* termination emits
* FlowTransformerSpec
* rework types to work with Java API
* rename and move things
* add scaladoc
Merge pull request #16287 from akka/wip-15236-transformer-patriknw

!str #15236 new transformer