First complete version of Kafka flowlet

Quick links:

README: https://github.com/caskdata/cdap-packs/blob/feature/kafka/cdap-kafka-pack/cdap-kafka-flow/README.md

BUILD: https://github.com/caskdata/cdap-packs/blob/feature/kafka/cdap-kafka-pack/cdap-kafka-flow/BUILD.md
Abstract base class for implementing consuming data from a Kafka cluster. This class serves as the generic base
to help implementing a flowlet that can poll from a specific Kafka version. Users should be extending from one
of the sub-classes of this class.