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.
I have not yet understood how it all works together, but this looks a bit too indirect and convoluted at first sight. I get the need for going around the stream in order to not complicate matters for the 99% case, but why is `oneHundredContinueRef` a function? There might be a good reason for this, and I’ll keep reading, but my spidey senses are tingling.