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.
Not sure whether I agree to this reasoning: assume that I am a convenience-biased user and just feed the entity to a file for writing it to disk, then onComplete I issue a 200. If there is a parsing error then the file will be created and partially written and the server will reply with a 400. Is this reasonable and expected? (it might be; I wouldn’t expect this but then again I am not a web developer)

Another question is whether a parsing error MUST always result in an error response.