Add configurable cluster domain support for non-standard Kubernetes environments

[odoucet]

This PR solves #6 (comment)

The operator hardcoded cluster.local as the DNS suffix, breaking deployments in Kubernetes clusters using custom domains (e.g., kube.prod, k8s.mycorp.local). This caused DNS resolution failures in Sentinel and Valkey HA configurations.

Changes:

• API (api/v1alpha1/types.go): Added optional ClusterDomain field to ValkeySpec with DNS-compliant validation pattern and default value cluster.local

• Binding logic (pkg/operator/binding.go): Replaced hardcoded DNS suffix with valkey.Spec.ClusterDomain in service hostname construction for both sentinel and non-sentinel modes

• Helm integration (pkg/operator/data/parameters.yaml): Passed clusterDomain parameter to Bitnami chart and updated TLS certificate DNS SANs to use configurable domain

• Documentation: Added usage example and field description to README

Usage:

apiVersion: cache.cs.sap.com/v1alpha1
kind: Valkey
metadata:
  name: valkey-prod
spec:
  clusterDomain: kube.prod
  replicas: 3
  sentinel:
    enabled: true

Generates DNS names: valkey-prod.namespace.svc.kube.prod instead of cluster.local

Backward compatible—existing deployments continue using cluster.local by default.

Disclaimer : This PR was largely written with Copilot/Claude 4.5

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[odoucet]

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