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.
I don't think we should completely disallow sending `HTTP/1.0` responses.
I bet there are applications out there which, for some weird reason, must be able to send a `HTTP/1.0` response.