+str,htc #16071, #16072 New Stream Tcp and Http API

* StreamTcp and Http extensions now return Flows and Sources that can be materialized later
* Flow can now be completed with another flow to be turned into a runnable flow
* Java side of things is missing so tests will fail
Nah, an `UndefinedSource` is a `Sink` to the user of the resulting `Flow`. Yes, head spinning. Maybe if the `userOut` was called `userOutgoing` and `userIn` `userIncoming`?