!str #15236 Replace Transformer with Stage

* replace all existing Transformer with Stage (PushPullStage)
* use Flow[ByteString, ByteString] as encoder/decoder transformer in http
* use the IteratorInterpreter for strict if possible
* emit then become
* emit then finish
* termination emits
* FlowTransformerSpec
* rework types to work with Java API
* rename and move things
* add scaladoc

akka-http-tests/src/test/scala/akka/http/server/directives/CodingDirectivesSpec.scala

@@ -507,4 +507,4 @@ class CodingDirectivesSpec extends RoutingSpec {
     be(Some(`Content-Encoding`(encoding))) compose { (_: HttpResponse).header[`Content-Encoding`] }
 
   def readAs(string: String, charset: String = "UTF8") = be(string) compose { (_: ByteString).decodeString(charset) }
-}
+}