ZOOKEEPER-5037: Add export and import commands to the CLI

[ckrumbein]

This PR adds two commands to the ZooKeeper CLI, export and import:

• "export <path> <filepath>" downloads the contents of a znode to an external file.
• "import <path> <filepath>" uploads the contents of an external file to a znode, replacing the znode's previous contents.

My team has been using both of these commands for some time. We are confident they are solid.