Add support for configuring Operational Backup on Blobs

[celloza]

Azure Blob backup supports Operational Backups: https://learn.microsoft.com/en-us/azure/backup/blob-backup-overview?tabs=operational-backup

This is supported in the AzureRM provider: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/data_protection_backup_policy_blob_storage

by specifying the operational_default_retention_duration property.

[celloza]

Sample support implemented in https://github.com/celloza/az-backup/tree/feature/support-operational-blob-backups