!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.
Yes, what this does is `activeFlow.join(hander).run()`.
BUT: "joining" a flow with another flow is not something that is readily understandable from the user perspective.
`handleWith` has a better connotation.

> I am confused by these active/passive server scenarios.

What exactly are you confused by?
Is there something in the scaladoc that we should improve?