Support `Intel TDX` instances on GCP

[HuijingHei]

Google Cloud Platform supports "Confidential Compute Instances" which are run on Intel TDX hardware nodes, see https://cloud.google.com/blog/products/identity-security/rsa-google-intel-confidential-computing-more-secure.

In order to fully support this VM option, the images need to be marked as TDX_CAPABLE.

Check list:

• Mark FCOS GCP images as TDX_CAPABLE.
• Add a test to make sure that FCOS runs on TDX_CAPABLE instances in GCP.

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Related AMD SEV-SNP issue, see:

• gcloud: Enable SEV_SNP_CAPABLE coreos-assembler#3547
• gcp: Support AMD SEV-SNP confidential instances coreos-assembler#3871 & mantle/platform/gcloud: fix confidential compute check coreos-assembler#3876

[jlebon]

Re-open for tests.

[HuijingHei]

Maybe click the wrong button?

[HuijingHei]

Add test coreos/fedora-coreos-config#3356, welcome to review, thanks!