[Feature] Blue/Green Cluster K8s upgrades - Nodepool

**Is your feature request related to a problem? Please describe.**
Customers often orchestrate our own nodepool upgrades for K8s versions and node images to have zero application down time and test the new nodepool/k8s version before switching over traffic/requests . Use the new version deployment to do smoke tests or metric based roll forward with the availability to rollback in case of failures/issues.

**Describe the solution you'd like**
AKS to orchestrate Nodepool and Control plane upgrades in a Blue-green fashion giving customers certain levers to rollback based on metrics, have bake time of the new version to do smoke testing and gracefully drain the older versions.