ZKFileDataSource - Data import handler ZooKeeper data source

Data import handler doesn't have a data source type to pull files from ZooKeeper. So config files for data import have to be synchronized manually across all nodes in a cluster for them to work. Since I'm still stuck with using DIH for the time being I made a small class that adds this functionality.

It works in the same way as FileDataSource does. It has the same encoding argument and basePath does the equivalent thing (if it's not set it assumes /configs/COLLECTION_NAME/FILENAME).