#19238 add detach operator and fix Detach stage

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

@@ -1299,6 +1299,20 @@ trait FlowOps[+Out, +Mat] {
     via(new Throttle(cost, per, maximumBurst, costCalculation, mode))
   }
 
+  /**
+   * Detaches upstream demand from downstream demand without detaching the
+   * stream rates; in other words acts like a buffer of size 1.
+   *
+   * '''Emits when''' upstream emits an element
+   *
+   * '''Backpressures when''' downstream backpressures
+   *
+   * '''Completes when''' upstream completes
+   *
+   * '''Cancels when''' downstream cancels
+   */
+  def detach: Repr[Out] = via(GraphStages.detacher)
+
   /**
    * Delays the initial element by the specified duration.
    *