Slight gaps in aks-node-controller flow

Context: We want to move our node provisioning process to aks-node-controller, as the "future" bootstrapping mechanism in Azure. I noticed some minor gaps in the controller's flow, raising those for consideration.

## Issue 1: Missing Cilium option

This one sounds trivial, unless I am missing something. In [GetNetworkPolicyType](https://github.com/Azure/AgentBaker/blob/main/aks-node-controller/helpers/utils.go#L55), there is no mapping for [Cilium](https://pkg.go.dev/github.com/Azure/azure-sdk-for-go/sdk/resourcemanager/containerservice/armcontainerservice/v9#NetworkPolicy). If I understand correctly, with Cilium turned on, then both `NetworkPolicy` and `NetworkDataplane` would be `cilium`. So it should be a matter of adding an enum value, mapping and output to script?

## Issue 2: Configuring NetworkPlugin correctly

Passing the correct value here took me some trial and error for the different CNI options. I assumed [GetNetworkPluginType](https://github.com/Azure/AgentBaker/blob/main/aks-node-controller/helpers/utils.go#L44) is sufficient, but it failed in real clusters for Azure CNI combos.

Empirically, this is what I found:
- Azure CNI overlay -> `none`
- Azure CNI overlay + Cilium -> `none`
- Azure CNI node subnet -> `azure`

Not 100% sure if that is correct, but nodes seem to join OK and network was working.
I assume this is one of those slow-moving items that "once you configure right, you touch every 1-2 years tops". Therefore, adding docs on how to pick up the value correctly from the start would be great.

As long as the project accepts contributions (and with some hand-holding), I'd be happy to open PRs for those changes, if you believe they make sense.