!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
withContext can be done without requiring 1-to-1 but that needs the lens-style approach, not the split-remerge approach. But please move that discussion to the relevant ticket.