+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
Aargh, you’re right, got tripped up walking around in circles. Yes, everything is fine. Apart from the signature of `Flow.apply` which should take a `FlowGraphBuilder => (Sink, Source)` (because in our drawings we always go left to right; alternatively I could argue that we always use `[In, Out]` types).