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/java/jdocs/stream/Main.java

@@ -4,9 +4,12 @@
 
 package jdocs.stream;
 
+import akka.actor.ActorSystem;
+
 // #main-app
 public class Main {
   public static void main(String[] argv) {
+    final ActorSystem system = ActorSystem.create("QuickStart");
     // Code here
   }
 }