Add combinedMat method to Source (#23809)

* Add combinedMat method to Source

* Fix formatting

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

@@ -448,6 +448,19 @@ object Source {
       combineRest(2, rest.iterator)
     })
 
+  /**
+   * Combines two sources with fan-in strategy like `Merge` or `Concat` and returns `Source` with a materialized value.
+   */
+  def combineMat[T, U, M1, M2, M](first: Source[T, M1], second: Source[T, M2])(strategy: Int ⇒ Graph[UniformFanInShape[T, U], NotUsed])(matF: (M1, M2) ⇒ M): Source[U, M] = {
+    val secondPartiallyCombined = GraphDSL.create(second) { implicit b ⇒ secondShape ⇒
+      import GraphDSL.Implicits._
+      val c = b.add(strategy(2))
+      secondShape ~> c.in(1)
+      FlowShape(c.in(0), c.out)
+    }
+    first.viaMat(secondPartiallyCombined)(matF)
+  }
+
   /**
    * Combine the elements of multiple streams into a stream of sequences.
    */