cli: fix unmarshalling of sev-snp attestation documents in `constellation verify`

[daniel-weisse]

Context

attDoc.Attestation.TeeAttestation is an interface, so json.Unmarshal can't parse data into it by default. https://github.com/edgelesssys/constellation/pull/3164 removed a custom type from the constellation verify code in favor of using vtpm.AttestationDocument. The custom type replaced the interface with a concrete *attest.Attestation_SevSnpAttestation to circumvent the parsing issue.

Proposed change(s)

• Implement a custom marshalling function to successfully parse data into attDoc.Attestation.TeeAttestation depending on attestation variant

Related issue

• Fixes https://github.com/edgelesssys/issues/issues/686

Checklist

• [x] Run the E2E tests that are relevant to this PR's changes
  • [x] verify gcp-sev-snp

[netlify[bot]]

✅ Deploy Preview for constellation-docs canceled.

Name	Link
🔨 Latest commit	0e714d8dd262b53c3ee3104561ca2b173900a82e
🔍 Latest deploy log	https://app.netlify.com/sites/constellation-docs/deploys/6670010ec3202d0008732b58

[github-actions[bot]]

Coverage report

Package	Old	New	Trend
cli/internal/cmd	40.90%	41.00%	:arrow_upper_right: