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
Returns the default value of the offset to start with when encountering a new broker for a given topic partition.
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By default, it is \{@code -2L}, which represents the earliest offset in Kafka. Sub-classes can override this to return a
different value (for example \{@code -1L}, which means the latest offset).