Add Akka.Streams.Kafka.Testkit to support other specs

[Arkatufus]

[Arkatufus]

All the changes to the *Test.cs are cosmetic changes, old tests are still using the old KafkaIntegrationTest base class. New ported tests should use the new KafkaSpec base class. The new abstracted fixture should allow for testing multi-broker kafka cluster.

Whats missing compared to the JVM implementation is fixture readiness check, they have methods to make sure that the brokers/cluster are ready before starting their test. Dotnet client does not have some of the admin features needed to query these states and the new fixture need to be enhanced with script injection to do these tests right in the container instances themselves.

[Aaronontheweb]

This is all meant to be internal right? Not for distribution to users?