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/java/akka/stream/io/SinkAsJavaSourceTest.java

@@ -33,8 +33,7 @@ public class SinkAsJavaSourceTest extends StreamTest {
   public void mustBeAbleToUseAsJavaStream() throws Exception {
     final List<Integer> list = Arrays.asList(1, 2, 3);
     final Sink<Integer, Stream<Integer>> streamSink = StreamConverters.asJavaStream();
-    java.util.stream.Stream<Integer> javaStream =
-        Source.from(list).runWith(streamSink, materializer);
+    java.util.stream.Stream<Integer> javaStream = Source.from(list).runWith(streamSink, system);
     assertEquals(list, javaStream.collect(Collectors.toList()));
   }
 }