XD-1329 Add Kafka source module

- Add kafka source module with consumer configurations
- Add `spring-xd-extension-kafka` subproject
  - Add Kafka module options mixin and  source module options metadata

Fix code review comments
  - Add kakfa zkconnect chroot path for kafka zk namespace
     - Allow the `zkconnect` property be overridden via modules.yml/kafka.properties
  - Update javadoc for StringDecoder
  - Update kafka inbound channel adapter's poller interval to `100ms` and
    max-messages per poll to `-1` which is unlimited.

Add `encoding` module option for string ValueDecoder

- Use `util:properties` for consumer-properties
  - Add config/kafka-consumer.properties to externalize the properites

Add Kafka Message transformer
  - Message transformer that transforms the kafka messages into
Map<Integer,Object> where key is partition index and value is payload.