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.
This might warrant a comment: the right branch means that 100-continue was expected but the application generated a response right away. Can this even occur? Should we not have pulled the 100-continue out of the response source at this point?