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.
Yes, this sounds good. The default case will then probably be that the request entity stream’s error will propagate and a 500 is sent, but that’s fair since the server did not care enough to do something else.