Support cluster zone in Gateway Discovery

[pmalek]

Problem statement

KIC currently (as of 3.0) doesn't take into account cluster zone (default: cluster.local) and in the Gateway Discovery mechanism it crates non fully qualified addresses (without the cluster zone).

There might be some situations where configuring the zone and including it in generated addresses would make sense.

This issue is a placeholder for the effort to support just that.

Relevant links:

Example how to change cluster's dns zone: https://stackoverflow.com/a/66106716

CoreDNS config explanation which mentions the default zone (cluster.local): https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/