Add PNI option to AKS and GKE imported clusters

[ryansann]

What kind of request is this (question/bug/enhancement/feature request):
enhancement

In order to support Project Network Isolation for generic imported clusters, there needs to be a UI option for Enable Project Network Isolation

For RKE clusters, this option appears in the Kubernetes Options section of the provisioning page. Something similar can be done for the generic import cluster page.

Checking this option, i.e. Enabling PNI, should set enableNetworkPolicy on the v3.Cluster object to true

Additional Context:
This was previously done for ember UI and needs to be implemented in dashboard, backend is in

Ember issue:

• Add Enable PNI Option to Generic Cluster Import rancher#33024

Backend issue:

• Allow PNI to be enabled for generic imported clusters rancher#33016

[catherineluse]

I have some questions about the requirements:

• Does this need to work on K3s, RKE2 and RKE1 clusters? I tried testing on imported K3s and RKE2 clusters and got a back end error that enabling it is not an option. I'm not sure if this is back end blocked or if I misunderstood the requirement
• Cody pointed out that the form for editing an imported K3s cluster actually already has a checkbox to enable PNI (even though the API throws an error if you try to enable it). It's under advanced options. Is this also where we would want to expose the option for RKE2?

I put screenshots in my draft PR rancher/ui#4815