Refactor Kafka credential handling into kafka-handler module

Currently the code for handling credentials and authentication for the Kafka storage handler is spread across hive-exec (https://github.com/apache/hive/blob/7cd3107a76d633ef5fae2ffb8ec16953ac968092/ql/src/java/org/apache/hadoop/hive/ql/exec/tez/DagUtils.java#L302) and kafka-handler (https://github.com/apache/hive/blob/7cd3107a76d633ef5fae2ffb8ec16953ac968092/kafka-handler/src/java/org/apache/hadoop/hive/kafka/KafkaUtils.java#L71) modules. This leads to unexpected dependencies to kafka (e.g., kafka-client) in hive-exec module and limits sharing opportunities of credential utilities present in KafkaUtil class.

The goal of this refactoring is threefold:

• centralize Kafka credential handling (in one module) opening the road to more code reuse and better encapsulation;
• remove kafka dependencies (e.g., kafka-client) from hive-exec module;
• facilitate unit testing and product stability.