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.
Unfortunately this won't work. We need to listen for a potential `EntityStreamError` element from the bypass, because the parser on the incoming side might detect a parse error in the request entity (e.g. because a chunk was not framed correctly). In these cases we want an error response to be rendered to the client even *before* or *instead* of the application having sent a response.