Implements actorRef source variant with backpressure #17610 (#26633)

* Implements actorRef source variant with backpressure #17610

* Small improvements to documentation and source #17610

* Small improvements to test #17610

* Small improvements to implementation and tests #17610

* Adds API for akka-typed #17610

* Adds ack sender and java api for typed #17610

akka-docs/src/main/paradox/stream/operators/Source/actorRefWithAck.md

@@ -0,0 +1,33 @@
+# actorRefWithAck
+
+Materialize an `ActorRef`; sending messages to it will emit them on the stream. The source acknowledges reception after emitting a message, to provide back pressure from the source.
+
+@ref[Source operators](../index.md#source-operators)
+
+@@@ div { .group-scala }
+## Signature
+
+@@signature [Source.scala](/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala) { #actorRefWithAck }
+@@@
+
+## Description
+
+Materialize an `ActorRef`, sending messages to it will emit them on the stream. The actor responds with the provided ack message
+once the element could be emitted alowing for backpressure from the source. Sending another message before the previous one has been acknowledged will fail the stream.
+
+@@@div { .callout }
+
+**emits** when there is demand and there are messages in the buffer or a message is sent to the `ActorRef`
+
+**completes** when the `ActorRef` is sent `akka.actor.Status.Success`
+
+@@@
+
+## Examples
+
+
+Scala
+:  @@snip [actorRef.scala](/akka-docs/src/test/scala/docs/stream/operators/SourceOperators.scala) { #actorRefWithAck }
+
+Java
+:  @@snip [actorRef.java](/akka-docs/src/test/java/jdocs/stream/operators/SourceDocExamples.java) { #actor-ref-imports #actor-ref-with-ack }