!htc #15716: 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
This should be solvable with an implicit that takes a higher-kinded type-class, but I agree that that is advanced type sorcery; thanks for the ticket!