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.
`absorbTermination` is only intended to be used from `onUpstreamFinish` or `onUpstreamFailure`. There will only be `onPull` afterwards.