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/FlowParallelismDocTest.java

@@ -19,19 +19,16 @@ import akka.stream.javadsl.*;
 public class FlowParallelismDocTest extends AbstractJavaTest {
 
   static ActorSystem system;
-  static Materializer mat;
 
   @BeforeClass
   public static void setup() {
     system = ActorSystem.create("FlowParallellismDocTest");
-    mat = ActorMaterializer.create(system);
   }
 
   @AfterClass
   public static void tearDown() {
     TestKit.shutdownActorSystem(system);
     system = null;
-    mat = null;
   }
 
   static class ScoopOfBatter {}