Add support for admin port on ZooKeeper

[liamgib]

Fixes #549

Motivation

We need to be able to access the ZooKeeper admin endpoint to perform automated snapshots to an external system.

Modifications

I have added the admin port to the list of Zookeeper ports, and made the relevant changes on the zookeeper-service.yaml & zookeeper-statefulset.yaml to use the admin port.

Verifying this change

• [ ] Make sure that the change passes the CI checks.

[liamgib]

Good point @lhotari, there is some security implications. Especially for existing clusters, where they might not be expecting the admin port being exposed.

I'll update the PR to make the field optional, and removed by default.