System wide materializer (#27491)

Introduces a materializer started through an extension and then an implicit
conversion for Scala turning an implicitly available ActorSystem into a
materializer. The Java APIs has been ammended with run-methods accepting
an ActorSystem.

akka-stream-tests/src/test/scala/akka/stream/FusingSpec.scala

@@ -10,8 +10,6 @@ import akka.stream.testkit.StreamSpec
 
 class FusingSpec extends StreamSpec {
 
-  implicit val materializer = ActorMaterializer()
-
   def actorRunningStage = {
     GraphInterpreter.currentInterpreter.context
   }