Support SEV_SNP confidential type selection on GCP

[bgartzi]

GCP compute API permits not only specifying whether a machine has to be confidential or not, but also determining the technology backing that up see [0]. This commit adds an api parameter into the GCPMachineProviderSpec to support that.

That way, we should be able to configure SEV_SNP machines as well as SEV machines as before.

[0] https://cloud.google.com/confidential-computing/confidential-vm/docs/supported-configurations

Upstream/CAPG PR: kubernetes-sigs/cluster-api-provider-gcp#1410

[openshift-ci]

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[openshift-ci]

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[openshift-ci]

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