Add cluster mode support

[qjsoq]

This pull request introduces a new, comprehensive Helm chart named valkey-cluster for deploying a clustered Valkey instance on Kubernetes.
This chart is designed to handle the complexities of a clustered setup by leveraging a StatefulSet and an intelligent initialization script.

What This PR Does:

• StatefulSet Deployment: Ensures stable network identifiers and ordered deployment, which is critical for a clustered database.
• Introduces Dynamic Per-Pod PVCs: Implements volumeClaimTemplates to automatically provision a unique PersistentVolumeClaim for each cluster node, ensuring data persistence and isolation.
• Automated Cluster Initialization: A new init-cluster.sh script (managed via cluster_config.yaml) is introduced. This script handles complex cluster bootstrapping, node discovery, and dynamic joining logic for new or replacement pods.
• Graceful Master Failover: Adds a default preStop lifecycle hook. This critical feature ensures that when a master pod is scheduled for termination (e.g., during a rolling update), it first triggers a controlled failover to one of its slaves before shutting down, minimizing downtime.
• Adds Headless Service: Creates the necessary headless service for stable network discovery between pods within the StatefulSet.
• Exposes Metrics Exporter: Adds a dedicated Kubernetes Service for the Prometheus metrics exporter sidecar. This makes cluster metrics easily discoverable and scrape-able for monitoring systems.
• Adds a PodDisruptionBudget to protect the cluster during voluntary disruptions.

How the Auto-Clustering Works
The core of the cluster logic resides in the init-cluster.sh script, which runs as a background process in the main container:

Peer Discovery: Each pod uses the Kubernetes headless service to discover its peers.

Joining an Existing Cluster: If a new pod starts and discovers an already healthy cluster, it uses the CLUSTER MEET command to join. It then intelligently finds a master with a deficit of replicas and assigns itself as a replica using CLUSTER REPLICATE.

Initial Cluster Creation: If no cluster is found, the first pod (-0) takes on the role of the initiator. It waits for all other pods to become ready and then bootstraps the entire cluster using the valkey-cli --cluster create command with the appropriate number of replicas.

Resiliency: The script can handle pods restarting by attempting to re-join the cluster and forget its old failed entry if necessary.

Here are some screenshots demonstrating the chart in action after a successful deployment.
For deployment these settings were set:

replicaCount: 6
metrics:
  enabled: true
dataStorage:
  enabled: true
  requestedSize: "8Gi"

1. Cluster Nodes Output

This shows the successful formation of a 6-node cluster (3 masters, 3 replicas).

2. Running Pods and PVCs

This confirms that the StatefulSet has successfully created all pods and their corresponding PersistentVolumeClaims.

3. Services