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-docs/src/test/scala/docs/stream/StreamTestKitDocSpec.scala

@@ -6,7 +6,6 @@ package docs.stream
 
 import akka.stream._
 import akka.stream.scaladsl._
-import akka.stream.testkit._
 import akka.stream.testkit.scaladsl._
 import scala.util._
 import scala.concurrent.duration._
@@ -16,8 +15,6 @@ import akka.pattern
 
 class StreamTestKitDocSpec extends AkkaSpec {
 
-  implicit val materializer = ActorMaterializer()
-
   "strict collection" in {
     //#strict-collection
     val sinkUnderTest = Flow[Int].map(_ * 2).toMat(Sink.fold(0)(_ + _))(Keep.right)