!str #19005 make groupBy et al return a SubFlow

A SubFlow (or SubSource) is not a Graph, it is an unfinished builder
that accepts transformations. This allows us to capture the substreams’
transformations before materializing the flow, which will be very
helpful in fully fusing all operators.

Another change is that groupBy now requires a maxSubstreams parameter in
order to bound its resource usage. In exchange the matching merge can be
unbounded. This trades silent deadlock for explicit stream failure.

This commit also changes all uses of Predef.identity to use `conforms`
and removes the HTTP impl.util.identityFunc.

akka-http-testkit/src/main/scala/akka/http/scaladsl/testkit/RouteTest.scala

@@ -110,7 +110,7 @@ trait RouteTest extends RequestBuilding with WSTestRequestBuilding with RouteTes
    * The result of the pipeline is the result that can later be checked with `check`. See the
    * "separate running route from checking" example from ScalatestRouteTestSpec.scala.
    */
-  def runRoute: RouteTestResult ⇒ RouteTestResult = akka.http.impl.util.identityFunc
+  def runRoute: RouteTestResult ⇒ RouteTestResult = conforms
 
   // there is already an implicit class WithTransformation in scope (inherited from akka.http.scaladsl.testkit.TransformerPipelineSupport)
   // however, this one takes precedence