Initial SourceWithContext, FlowWithContext, FlowWithContextOps.

(based on https://github.com/akka/akka/compare/master...jrudolph:jr/withContextOps)

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

@@ -8,6 +8,7 @@ import java.util
 import java.util.Optional
 
 import akka.actor.{ ActorRef, Cancellable, Props }
+import akka.annotation.ApiMayChange
 import akka.event.LoggingAdapter
 import akka.japi.{ Pair, Util, function }
 import akka.stream._
@@ -3466,4 +3467,11 @@ final class Source[Out, Mat](delegate: scaladsl.Source[Out, Mat]) extends Graph[
    */
   def log(name: String): javadsl.Source[Out, Mat] =
     this.log(name, ConstantFun.javaIdentityFunction[Out], null)
+
+  /**
+   * API MAY CHANGE
+   */
+  @ApiMayChange
+  def startContextPropagation[Ctx](extractContext: function.Function[Out, Ctx]): SourceWithContext[Ctx, Out, Mat] =
+    new javadsl.SourceWithContext(scaladsl.SourceWithContext(this.asScala).mapContext(extractContext.apply))
 }