operator: make minio endpoint cluster domain configurable - #3702

Fixes #2895

Problem

The operator's Helm chart hardcodes .svc.cluster.local when building the in-cluster Minio endpoint:

operator/templates/manager/manager.yaml:78
value: "{{ .Release.Name }}-minio.{{ .Release.Namespace }}.svc.cluster.local"

On clusters configured with a non-default cluster domain (e.g. kubelet started with --cluster-domain=mycorp.internal), this hostname doesn't resolve, and the operator can't reach its own bundled Minio.

Fix

Adds a clusterDomain value to operator/values.yaml, defaulting to cluster.local (no change to existing behavior), and uses it in the endpoint template instead of the hardcoded literal.

Note on prior attempt

I saw that #2921 previously tried to address this issue and was closed — that PR referenced a nonexistent image and changed the Minio image tag/version instead of actually touching the hardcoded endpoint, so it didn't fix the reported problem. This PR only touches the clusterDomain value and the one template line that builds the endpoint string; it doesn't change the Minio image, tag, or any chart dependency.

Testing

• Added a helm-unittest case (allows overriding the cluster domain used for the minio endpoint) asserting S3_ENDPOINT reflects a custom clusterDomain.
• Confirmed the existing configures manager deployment for minio test still passes with the default value (cluster.local), preserving backward compatibility — and verified it actually catches a regression by temporarily breaking the default and re-running.
• Rendered the chart directly with helm template both with and without --set clusterDomain=... to confirm the output is correct in both cases:

  $ helm template operator --set clusterDomain=mycorp.internal --show-only templates/manager/manager.yaml | grep S3_ENDPOINT
  value: "release-name-minio.default.svc.mycorp.internal"
  
  $ helm template operator --show-only templates/manager/manager.yaml | grep S3_ENDPOINT
  value: "release-name-minio.default.svc.cluster.local"
  

• Updated operator/README.md's generated values table with the new clusterDomain entry.

I wasn't able to verify this against a live cluster with a custom cluster domain, but the chart-level behavior (default preserved, override applied) is covered by the test above.