+str #15034 Add Flow javadsl

* First stab for early review only

This is how it looks like:

```scala
    final ActorSystem system = ActorSystem.create("Sys");
    final MaterializerSettings settings = MaterializerSettings.create();
    final FlowMaterializer materializer = FlowMaterializer.create(settings, system);
  
    final String[] lookup = { "a", "b", "c", "d", "e", "f" };
    final java.lang.Iterable<Integer> input = Arrays.asList(0, 1, 2, 3, 4, 5);
    Flow.create(input).drop(2).take(3).
      map(new Function<Integer, String>() {
        public String apply(Integer elem) {
          return lookup[elem];
        }
      }).
      filter(new Predicate<String>() {
        public boolean defined(String elem) {
          return !elem.equals("c");
        }
      }).
      grouped(2).mapConcat(new Function<java.util.List<String>, java.util.List<String>>() {
        public java.util.List<String> apply(java.util.List<String> elem) {
          return elem;
        }
      }).fold("", new Function2<String, String, String>() {
        public String apply(String acc, String elem) {
          return acc + elem;
        }
      }).foreach(new Procedure<String>() {
        public void apply(String elem) {
          System.out.println(elem);
        }
      }).consume(materializer);
```

And in Java8:

```java
    final ActorSystem system = ActorSystem.create("Sys");
    final MaterializerSettings settings = MaterializerSettings.create();
    final FlowMaterializer materializer = FlowMaterializer.create(settings, system);
  
    final String[] lookup = { "a", "b", "c", "d", "e", "f" };
    final java.lang.Iterable<Integer> input = Arrays.asList(0, 1, 2, 3, 4, 5);
    Flow.create(input).drop(2).take(3).
      map(elem -> lookup[elem]).
      filter(elem -> !elem.equals("c")).
      grouped(2).
      mapConcat(elem -> elem).
      fold("", (acc, elem) -> acc + elem).
      foreach(elem -> System.out.println(elem)).
      consume(materializer);
```