[Provision alignment] Add Duo Nano support to new namespace provision API endpoint

Problem

Since the start of [Future of Provisioning] Alignment between GitL... (&16131 - closed), Duo Nano was added to a subscription's purchased add-ons. This includes an update to the namespace's add-on purchases API endpoint which is used via the Gitlab::AddOnPurchases::BulkSyncService.

There's a new namespace provisioning API endpoint that was introduced for the mentioned epic. It was not not updated to support Duo Nano since there's no usage for the new endpoint yet.

Proposal

Add Duo Nano to the new provision API endpoint to support the provisioning of it via the new single namespace provision workflow.

This change should also be added before enabling the feature flag to rollout the change for [Provision alignment] Replace current syncs wit... (customers-gitlab-com#12386 - closed).

Result

Duo Nano add-on provisioning via the new single namespace provision workflow.