Unable to access ZooKeeper admin port

[liamgib]

Is your feature request related to a problem? Please describe.
We need to be able to access the ZooKeeper admin endpoints to perform snapshots, and administrative actions. The Helm chart currently does not expose this port (9990) on the service, or statefulset specification.

Describe the solution you'd like
Allow the user to specify the admin port number in the Helm values to expose the admin port on the service.

Describe alternatives you've considered
Considered changing the admin port number to one that is not commonly used, but we're using all other ports.

Additional context
This is required so we can get the zxid of each ZooKeeper instances, and then perform a snapshot on the highest instance.