azure_kubernetes_service: add management plane operations

- GetNodePoolNames: lists current AKS node pools
- CreateNodePoolAsync: creates nodepool, returns np_succeeded handle
- DeleteNodePoolAsync: deletes nodepool, returns np_gone handle
- UpgradeNodePoolAsync: upgrades nodepool, returns np_succeeded handle
- UpdateClusterAsync: toggles tag for non-destructive update
- WaitForOperation: polls nodepool/cluster until terminal state
- ResolveNodePoolVersions: auto-detects initial/target versions
- Retry logic via vm_util.Retry (max 5, 30s interval)

Tested:
- Existing AKS test suite passing
- pyink + lint-diffs clean

Author: Ashish Suneja