Add configs for deploying keydb to HA cluster

[mutambaraf]

Part of : UserOfficeProject/issue-tracker#1325

1. Deploy a centralized KeyDB cache layer across three Kubernetes clusters (PROXMOX, STFC Cloud, ISIS VMs) using a full mesh multi-master replication topology.
2. Each cluster runs a single KeyDB instance that actively replicates to and from its two peers, providing cross-cluster cache consistency with no single point of failure.
3. Kustomize base manifests cover the Deployment, ConfigMap, Services, and replication wiring; per-cluster overlays patch environment-specific node selectors and replica endpoints.
4. Prometheus scraping is enabled on all instances via pod annotations for out-of-the-box observability.