Akka.Streams.Kafka API re-organization

[Aaronontheweb]

Going to write a lengthier description here once I take a full inventory of all the problems, but after the epic we did for v1.5.39 it's really clear that the Akka.Streams.Kafka package suffers from an excessively high number of namespaces, public extension methods on non-Akka.NET APIs (a no-no, generally), and just disorganization generally.

We're going to right this ship in v1.6 and basically re-organize the APIs so it's easier to find things you actually want to use.