+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
This looks inverted to me: a Flow looks more like a `(Sink, Source)` to me than the other way around.

A different thought is that `userOut` for me means “where the user puts their data”, but I guess this is not the only possible interpretation.