[Feature] Fleet: Multi-cluster updates cleanly manage AKS Automatic updates.

As a fleet administrator I want to be able to delegate automated cluster updates for my AKS Automatic clusters to Fleet Manager so that I can be confident that all my AKS clusters are updated through the same mechanism.

**Additional context**

AKS Automatic clusters can be added to a Fleet Manager, and Fleet Manager can control cluster upgrades for AKS Automatic clusters included in update runs.

However, as cluster-level auto-upgrade can't be disabled for AKS Automatic clusters, there is currently no way to ensure **only** Fleet Manager performs the upgrade. This means, Fleet Manager **or** AKS Automatic itself, might perform the upgrade, leading to clusters that may be updated outside of a Fleet Manager Update Run or Auto-upgrade.