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.
> Another question is whether a parsing error MUST always result in an error response.

You could also just abort the connection. As Mathias said: parsing errors of this kind are HTTP transport layer errors which leave the connection in a undetermined state. Many clients won't even be able to read any response when the server writes a response that the client doesn't expect at that time (i.e. while still sending the request).