!htc #15726: Akka HTTP with the new DSL

/cc @jrudolph @sirthias 

TODO:
 - [X] Port http-core
 - [x] Port rest of http
 - [ ] Port to the new IO abstraction
 - [x] get rid of unnecessary materializer parameters
yes, that is what I mean (sorry that I was unclear)

    final case object CancelDrain extends SimpleDrain[Any]