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
Should we set this as the location of our own ZK in the modules.yml defaults layer?

Also, kafka is pretty messy in its ZK usage, creating several dirs at the root. So although this is the kafka default, should we consider `localhost:2181/kafka` or maybe `localhost:2181/xd/kafka` ?