[zookeeper] Add support for custom servers list

[nenych]

Description of the change

My changes allow overriding the servers list in the zookeeper config.

Benefits

We can create a custom list of servers that allows us to combine 2 helm releases for the migration purpose, or more importantly, we can create custom services with the ClusterIp instead of headless for static IPs in the topology. This allows us to have no downtime (or at least smaller) on the node pool update or something else.

Possible drawbacks

No behavior changes without setting the new value.

Applicable issues

• fixes #

Additional information

Checklist

• Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• Variables are documented in the values.yaml and added to the README.md
• Title of the pull request follows this pattern [<name_of_the_chart>] Descriptive title
• All commits are signed and verified (required as of January 22, 2026 - see CONTRIBUTING.md)