retentionPolicy delete fails against S3-compatible storage with limited Multi-Object Delete support (NetApp ONTAP)

Our retention policy enforcement fails on every cycle against a NetApp ONTAP S3 backend:

"Could not delete backup 20260623T104935: An error occurred (AccessDenied) when calling the DeleteObjects operation: Access Denied"

We isolated the cause: barman-cloud-backup-delete uses the S3 Multi-Object Delete (bulk) API. A single `aws s3api delete-object` on the same key with the same credentials succeeds, but `aws s3api delete-objects` (bulk) on the identical key fails with AccessDenied, even with a correctly scoped bucket policy. This appears to be a known limitation of some non-AWS S3 implementations (similar reports: [cloudnative-pg/barman-cloud#149](https://github.com/cloudnative-pg/barman-cloud/issues/149) against MinIO).

barman-cloud-backup-delete already supports --batch-size, which set to 1 avoids the bulk delete path entirely and fixes it. However, the ObjectStore CRD has no way to pass extra args to the delete/retention command - additionalCommandArgs only exists for backup, restore, and WAL archive/restore.

Request: expose a field (e.g. deleteAdditionalCommandArgs) that gets passed to barman-cloud-backup-delete, so --batch-size (and similar flags) can be configured for retention enforcement, same as the other operations already support.

Finde die Definition der ObjectStore CRD und den Einstiegspunkt, der die Argumente für barman-cloud-backup-delete zusammenstellt, und vergleiche anschließend, wie additionalCommandArgs für Backup-, Restore- und WAL-Operationen behandelt wird. Füge den delete-spezifischen Konfigurationspfad hinzu, stelle sicher, dass --batch-size=1 die Durchsetzung der Aufbewahrung erreicht, und führe die relevanten CRD- und Controller-Tests aus.

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