=str #16095 make Actor{Publisher,Subscriber} as {Source,Sink}

* Props{Source,Sink} creates an actor from props as a child
of StreamSupervisor actor and wraps it into {Publisher,Subscriber}
* ActorRef{Source,Sink} wraps a given ActorRef into {Publisher,Subscriber}
* In both cases user actor should one of akka.stream.actor.Actor{Publisher,Subscriber}

Fixes #16095
the question is if we should provide both variants? What do you think of only providing the props variant, its managed by the "flow". If there is a need to create the actor elsewhere its something external to the "flow" and can then be treated as a raw Publisher/Subscriber. That can of course also be connected to the flow with the Source from Publisher factory. WDYT?