Add Kafka sink resource compatibility level attrs

[bobbyiliev]

Fixes: #587

This adds the new VALUE|KEY COMPATIBILITY LEVEL attribute as a config option to the avro format and will generate the following DDL when the attributes are set:

CREATE SINK compatibility_level_sink
  IN CLUSTER <some_cluster>
  FROM <source, table or mview>
  INTO KAFKA CONNECTION kafka_connection (
    TOPIC 'test_avro_topic',
  )
  FORMAT AVRO USING CONFLUENT SCHEMA REGISTRY CONNECTION csr_connection (
    KEY COMPATIBILITY LEVEL 'BACKWARD',
    VALUE COMPATIBILITY LEVEL 'BACKWARD_TRANSITIVE'
  )
  ...