+str #15736 Initial version of JavaDSL for akka-streams (NOT complete)

Missing features are:
+ be able to use all junctions from Java
* re-think akka.stream.javadsl.japi (because variance)
+ better docs
+ enable all tests
+ Partial flow graph as well as GraphFlow tests
+ enabled more FlowTest tests
+ fixed naming of internal classes (no Java in class names)
+ added converters for UndefinedSink/Source
Merge pull request #16047 from ktoso/wip-java-streams-ktoso

+str #15736 EARLY WIP akka-streams JavaAPIs