Expose NFS CSI plugin toggle in DOKS cluster create/update

[kgautam676]

Exposing NFS CSI plugin for DOKS clusters on create/update requests.

Merged godo and OpenAPI changes

doctl kubernetes cluster create example_cluster_name --enable-nfs-csi-plugin=true
doctl kubernetes cluster update example-cluster --enable-nfs-csi-plugin=true

[kgautam676]

Scenarios tested locally with PR binary :

• Updating a cluster to enable NFS CSI plugin in a non available region :

./doctl kubernetes cluster update a1648cac-acc1-4dcb-b360-c2ec3e6de05e \
 --enable-nfs-csi-plugin=true
Error: PUT https://api.digitalocean.com/v2/kubernetes/clusters/a1648cac-acc1-4dcb-b360-c2ec3e6de05e: 422 (request "92c47053-4c6c-44f8-810f-8f495060c949") validation error: NFS CSI Driver is not available in region "lon1"

• Created the cluster with plugin enabled
• Updated an existing cluster and enabled the plugin.
• Updating an existing cluster to disable the plugin and verified all relevant resources removed.

[skumarc-do]

Changes LGTM 👍
Nit: Could we add cases for flag omitted and explicit =false?