Feature-1: Add backup Template

[doulikecookiedough]

Hi @artntek - this was blocking me from testing so I made some changes locally to test - it seems to work so I committed the changes and opened a PR. Can you please take a look and let me know if this PR resolves issue?

Summary of Changes:

• Added backup section with documentation in values.yaml
  • By default, we are going to use volumeSnapshots as the backup method with the csi-cephfsplugin-snapclass class
• Updated template pg_cluster.yaml with section to pick up user specified backup values
• I've tested these changes locally and it appears to be working:

  $ kubectl apply -f '/Users/doumok/Code/vegbank2/helm/backup/scheduled-backup.yaml' -n vegbank-dev --context=dev-k8s
  scheduledbackup.postgresql.cnpg.io/vegbankdb-scheduled-backup created
  
  $ kubectl get scheduledbackup -n vegbank-dev --context=dev-k8s
  NAME                         AGE   CLUSTER          LAST BACKUP
  vegbankdb-scheduled-backup   57s   vegbankdb-cnpg   57s
  
  $ kubectl get backups
  NAME                                        AGE   CLUSTER          METHOD           PHASE       ERROR
  vegbankdb-scheduled-backup-20251107221536   12s   vegbankdb-cnpg   volumeSnapshot   completed 

[artntek]

Awesome - thank you for doing this!