Barman Cloud Plugin installed on every cluster, with a flag-gated backup migration

The Helm chart now installs the Barman Cloud Plugin for the CloudNativePG operator as part of the standard platform release. A new opt-in value (default off) lets deployers flip their CNPG clusters (tuist and tuist-ops) from the deprecated in-tree barmanObjectStore backup path to the Barman Cloud Plugin, wiring up the matching ObjectStore resource, plugins list, and plugin-method ScheduledBackup so WAL archiving continues against the existing archive prefix. On upgrade with the flag left off, nothing about backups changes; the plugin just rides along ready for a later per-env cutover.