!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.
We do have the `UndefinedSource` and `UndefinedSink` types but that gets messy pretty quickly. I think we need a common  trait for something with one or two open ends, that you could easily hook into. I've opened a ticket for it #15963.