+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
I'd suggest to not spend too much time here.
I'm currently working on #16162, which will rework the outside HTTP API once more completely.
So I'd consider this a purely intermediate API state here.