Support for the ONTAP volume "-is-preserve-unlink-enabled" flag with the ontap-nas backend

[arndt-netapp]

The ONTAP volume setting -is-preserve-unlink-enabled is required for Kafka over NFS use cases. This feature implements a Trident PVC setting to enable this ONTAP volume setting with the ontap-nas driver.

To enable this setting with an ontap-nas backend, use the following in the backend definition defaults:

"defaults": {
    "preserveUnlink": "true",
}

Alternatively, this setting can also be enabled on a per PVC basis with the following PVC annotation:

annotations: trident.netapp.io/preserveUnlink: "true"

If no value is given for preserveUnlink in the backend or PVC definition, it will default to "false".

While the is-preserve-unlink-enabled volume setting was first introduced with ONTAP 9.12.1, this feature in Trident requires the use of ONTAP 9.15.1 or higher where Trident uses the ONTAP REST API by default. The is-preserve-unlink-enabled volume setting in ONTAP is not currently exposed by the ONTAP REST API, so we use the