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 is another option we considered.
One could argue that parse errors on the initial part of the request are nothing that the application has to be aware of.
However, simply closing the stream will also create a race with the error coming in across the bypass.