[Feature] Support the label node-role.kubernetes.io/CUSTOM to allow standard tools like K9s kube-stat-metrics to correctly assign the node role

[abossard]

Related issue in the Kube State Metrics project: kubernetes/kube-state-metrics#2474

Is your feature request related to a problem? Please describe.
Kube State Metrics relies on node-role.kubernetes.io labels to understand what role a node belongs to. These are official labels from the K8s project.
Monitoring tools that rely on kube state metrics won't populate the node role attributes due to the lack of that.
Open Shift and other platforms allow to set your custom node-roles as well, which nicely integrates them into all tools that base their role on kube state metrics.

E.g. node with GPU can have:
node-role.kubernetes.io/GPU=true

It's preferable to have these standardized labels, since it makes it easier to migrate from and to different K8s providers.

Also Kube State Metrics doesn't allow customization on where to get the node role from.

Describe the solution you'd like
I would like that AKS allows setting custom node-role.kubernetes.io/* labels, als long as they don't interfere with existing ones.

Describe alternatives you've considered
An alternative would be that Kube State Metrics gets changed so that e.g. it would allow getting the node-role labels from a different label namespace. But since the node-role labels are well documented with K8s, I think it's preferrable if AKS adds the labels.

Additional context
See the attached issue for some more details kubernetes/kube-state-metrics#2474