feat(sks): add Karpenter credential rotation and active nodepool template lookupJbruinink/sks karpenter rotation nodepool templates

[jbruinink]

Description

sks: add Karpenter credential rotation and active nodepool template lookup.

This PR adds two new SKS CLI commands:

• exo compute sks rotate-karpenter-credentials <CLUSTER-NAME|ID> [-z ZONE]
• exo compute sks active-nodepool-templates <KUBERNETES-VERSION> [--variant standard|nvidia] [-z ZONE]

Details:

• rotate-karpenter-credentials mirrors existing SKS credential rotation commands (CCM/CSI), triggers the SKS API rotation operation, and waits for operation success.
• active-nodepool-templates queries active SKS nodepool templates for a Kubernetes version and returns both template ID and resolved template name.
• By default, active-nodepool-templates queries both standard and nvidia variants; --variant limits lookup to one variant.
• Both commands are registered under exo compute sks and appear in help output.

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block)
• Testing

Testing

• go test ./cmd/compute/sks
• go run . compute sks --help
  • verified active-nodepool-templates and rotate-karpenter-credentials are listed
• Manual validation:
  • deleted the SKS Karpenter IAM key for a test cluster
  • verified the nodepool was not longer Ready
  • rotated credentials via rotate-karpenter-credentials
  • verified a new Karpenter key was issued
  • verified the nodepool returned to Ready