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.
The problem is that I need to get a hold of the `ActorRef` of the actor powering the `Source[OneHundredContinue.type]` *inside* of a flow (this stage here). As long as #16168 is not cleared I need to use an ugly side channel, which receives the `ActorRef` *after* materialization.