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.
We need to carry a signal from the demand side of one stream to a trigger in a completely independent stream.
Going through a side channel (as done here) appears to be the easiest and most efficient solution.