!htp #16162 Upgrade server- and client-side HTTP APIs to "final style"

This PR clears #16162.
It currently builds on #16383, so ignore Björn's commit in your review.

As a temporary measure I have added a stub TCP API as described by #16166. This `TempTcp` stuff will go away when #16166 is cleared.
I agree that for Tcp there doesn't really seem to be a need for this, but I think Http has. You could definitely have Http wire up the `Flow[HttpResponse, HttpRequest]` in the ServerBinding, but there it doesn't feel as intuitive since the input/output thing feels backwards. Here people don't have to think about it since they're the same type.