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.
If you scroll down one screen you'll see a very large comment explaining 100-continue support in detail.
Among other stuff it also says:

> The application can therefore determine itself whether it wants the client to send the request entity by deciding whether to look at the request entity data stream or not. If the application sends a response *without* having looked at the request entity the client receives this response *instead of* the `100 Continue` response and the server closes the connection afterwards.

Should I add a pointer to the other file here?