#19440 replace Scala Future usage with CompletionStage in javadsl

This entails:

  * adding akka.pattern.PatternCS.* to enable ask etc. with
    CompletionStage
  * changing RequestContext to offer an ExecutionContextExecutor for the
    CompletionStage.*Async combinators
  * splitting up akka.stream.Queue for JavaDSL consistency

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -23,6 +23,8 @@ import scala.collection.immutable
 import scala.concurrent.duration.{ FiniteDuration, _ }
 import scala.concurrent.{ Future, Promise }
 import akka.stream.impl.fusing.Buffer
+import java.util.concurrent.CompletionStage
+import scala.compat.java8.FutureConverters._
 
 /**
  * A `Source` is a set of stream processing steps that has one open output. It can comprise
@@ -230,6 +232,15 @@ object Source {
   def fromFuture[T](future: Future[T]): Source[T, NotUsed] =
     fromGraph(new FutureSource(future))
 
+  /**
+   * Start a new `Source` from the given `Future`. The stream will consist of
+   * one element when the `Future` is completed with a successful value, which
+   * may happen before or after materializing the `Flow`.
+   * The stream terminates with a failure if the `Future` is completed with a failure.
+   */
+  def fromCompletionStage[T](future: CompletionStage[T]): Source[T, NotUsed] =
+    fromGraph(new FutureSource(future.toScala))
+
   /**
    * Elements are emitted periodically with the specified interval.
    * The tick element will be delivered to downstream consumers that has requested any elements.