!hco #15716: Use new flow DSL in the Http implementation

Work In Progress

First attempt at using the new DSL internally in http. Probably not all uses of Flow has been removed yet, but they are gone from the main pipelines. I have not attempted to solve any of the DSL quirks yet, this is the time to discuss them.
Couldn't this be a partial flow graph that gets materialized later? I know that we are missing convenience types for a graph with one open sink/source or both a sink and a source.