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.
What this means is that the application might send a response without having looked at the request entity, i.e. the client will not receive a `100 Continue` response but something else which the application has provided. In these cases we close the connection in order to circumvent the potential ambiguity for what will be read next on the incoming side.