Add server-side support for `Expect: 100-continue`, other improvements

This PR improves a number of things in akka-http-core and adds server-side support for `Expect: 100-continue` (#15799).

The client-side support depends on #16336 and is therefore postponed for the time being.
This write is not properly published to the materialized flow, which will be executed on a different thread. Spidey senses were right …