Support custom annotations on the ScheduledBackup recource

[KazW]

Support for custom annotations on the ScheduledBackup resource would be a very useful feature to add to the cluster chart. It's useful because when the Helm release is uninstalled, any backups created by it can be left behind by adding the helm.sh/resource-policy: keep annotation to the ScheduledBackup resource.

The wal and base backups are left in S3 when the ScheduledBackup resource is deleted, but the associated Backup resources are deleted. It'd be nice to be able to keep both.

This would also enable using the backup method when reinstalling the chart with the same release name and values while passing the --take-ownership flag to helm.