+str #24229 move SinkRef / SourceRef to akka.stream

+str #24229 remove protobuf changes, which do not need to be made in this PR

docs

moved things

config object

subscription timeout confifmed working, also, attributes

document attributes for sub timeout

tests for the source also failing when it should

additional demand test

implemented protection from materializing "in cycles"; would be nice in
types but that breaks the niceness of use of the types
SinkRef/SourceRef...

cleanup

no idle timeout built in, can use the Timeout stages

more docs

simplest change to prevent exposing SinkRef => SourceRef => SinkRef cycle

Things to decide:
 * is it ok to require using `getSource` / `getSink` as Java API, is there better naming?
 * where should the constructors go? I'd say just in regular javadsl/scaladsl `Source`/ `Sink` objects

move constructors to {javadsl,scaladsl}.{Source,Sink} companion objects

Remove now useless "canMaterialize" field

Separate stage (implementation) from ref (wrapped actor ref) to make it clearer what is serialized

Clarify that partner refs are not optional in on-the-wire interfaces

minor cleanup in SourceRefStage

Renamed the stages but questionable if that really helps ;)

cleanups, better docs

cleanup, fix docs compilation

fix mima

got rid of Futures in the materialized values of stream refs

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

@@ -4,7 +4,8 @@
 package akka.stream.scaladsl
 
 import java.util.concurrent.CompletionStage
-
+import akka.util.ConstantFun
+import akka.{ Done, NotUsed }
 import akka.actor.{ ActorRef, Cancellable, Props }
 import akka.stream.actor.ActorPublisher
 import akka.stream.impl.Stages.DefaultAttributes
@@ -24,6 +25,12 @@ import scala.compat.java8.FutureConverters._
 import scala.concurrent.duration.FiniteDuration
 import scala.concurrent.{ Future, Promise }
 
+import akka.stream.impl.streamref.SourceRefStageImpl
+import akka.stream.stage.{ GraphStage, GraphStageWithMaterializedValue }
+import akka.util.OptionVal
+
+import scala.compat.java8.FutureConverters._
+
 /**
  * A `Source` is a set of stream processing steps that has one open output. It can comprise
  * any number of internal sources and transformations that are wired together, or it can be
@@ -601,4 +608,15 @@ object Source {
    */
   def unfoldResourceAsync[T, S](create: () ⇒ Future[S], read: (S) ⇒ Future[Option[T]], close: (S) ⇒ Future[Done]): Source[T, NotUsed] =
     Source.fromGraph(new UnfoldResourceSourceAsync(create, read, close))
+
+  /**
+   * A local [[Sink]] which materializes a [[SourceRef]] which can be used by other streams (including remote ones),
+   * to consume data from this local stream, as if they were attached in the spot of the local Sink directly.
+   *
+   * Adheres to [[StreamRefAttributes]].
+   *
+   * See more detailed documentation on [[SinkRef]].
+   */
+  def sinkRef[T](): Source[T, SinkRef[T]] =
+    Source.fromGraph(new SourceRefStageImpl[T](OptionVal.None))
 }