!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
I was merely commenting on the finding that the graph here will be superfluous, which might not be the case. And to answer the original point: both Flow declarations can be moved outside of the graph, resulting in the same code layout as Johannes proposes (modulo the encapsulation of the graph within its own method).