!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
if `FutureDrain` is considered used a lot (outside test code) I'm sure we can add it as `toFuture` and I also prefer to rename those to `HeadFutureDrain` and `toHeadFuture`.

For test code you can always add an implicit conversion for `toFuture`.