operator: use GCP REST API for instance templates

[msanft]

Context

GCP upgrades were broken previously as new nodes didn't use the correct instance types. This was caused by the Protobuf-based GCP Go SDK isn't aware of the confidential_instance_type field, which thus wasn't copied when creating a new instance template during an upgrade. This can be circumvented by using the REST API implementation, which is aware of the field.

Proposed change(s)

• Use the GCP REST API (the corresponding Go SDK, really) for managing instance templates during an upgrade.

Additional info

• AB#4660
• Instance template created before this change is missing confidential_instance_type
• Instance template created after this change has it set.

Checklist

• Run the E2E tests that are relevant to this PR's changes
  • GCP (SEV-SNP) upgrade test
• Add labels (e.g., for changelog category)
• Is PR title adequate for changelog?
• Link to Milestone

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[msanft]

From my manual testing, it seems that this patch is not sufficient to make SEV-SNP upgrades on GCP work again. Anyhow, I verified that this fix alone is correct by checking that instance templates now use the correct confidential instance type, so I think this fix should be merged in either case.

[github-actions]

Coverage report

Package	Old	New	Trend
operators/constellation-node-operator/internal/cloud/gcp/client	43.90%	43.70%	↘️