Add Source and SinkKafkaConnector super classes

[VictorKuenstler]

Currently, the single message transformers work for sink connectors. But the single message transformers should be handled differently for source and sink Kafka connectors. Using them for source connectors requires us to override get_topics and get_routes for a new connector type.

We should introduce superclasses for sink and source Kafka Connectors to make ease development of new connectors.