feat: add common pod labels support

[suraj2410]

This PR introduces common pod labels support for ERPNext deployments, allowing the same labels to be applied to all pods across all deployments in a single Helm release.

What's included

• ✅ Add commonPodLabels helper template for user-defined pod labels
• ✅ Update all deployment templates to include common pod labels
• ✅ Add commonPodLabels configuration to values.yaml with examples
• ✅ Preserve all existing pod labels without modification

Key Features

• Universal Application: Labels are automatically applied to all ERPNext deployments
• Non-intrusive: All existing pod labels remain completely untouched
• Backwards Compatible: Works with empty {} or missing commonPodLabels configuration
• Flexible: Supports any number of custom key-value label pairs

Usage Example

commonPodLabels:
  environment: production
  team: platform
  cost-center: engineering
  monitoring: enabled
  app-version: v16.8.0

Backward compatibility

• ✅ No existing values or workflows are broken
• ✅ All original pod labels preserved exactly as they were
• ✅ Upgrading users can opt in to common pod labels without changing current setups
• ✅ Works with empty configuration
  Testing
  All deployments (nginx, gunicorn, socketio, scheduler, workers) will now include:
• Existing labels: app.kubernetes.io/name, app.kubernetes.io/instance, app.kubernetes.io/app: frappe (untouched)
• Custom labels: Your common labels from values.commonPodLabels
  Fixes common requirement for applying organization-wide labels to Kubernetes resources.

[ismxilxrif]

@suraj2410 this is actually good, how about putting it under a key called global? we can later add other things such as annotations, tolerations etc for the whole deployment