Feature Request: Support arbitrary ZooKeeper config

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Feature Description

Add ability to add arbitrary config lines to ZooKeeper config via zkctl(d)

Use Case(s)

Currently zkctl(d) writes ZK config according to a template in the Vitess repo (zoo.cfg). There is no clean way to customize this config.

For example, we find it useful to add the following configs in our setup:

tcpKeepAlive=true
autopurge.snapRetainCount=10
autopurge.purgeInterval=24

Today we do this by running zkctl init, then modifying the config, then proceeding with other zkctl commands. This feels hacky. Given zkctl is writing the config, it would be nice if zkctl itself supported this.

PR to follow.

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Thank you @mattlord & team!