Add checking of TCB version when checking a SNP attestation

[cjen1-msft]

SNP attestation reports are checked by verify_snp_attestation_report but only validates that the TCB in the attestation report matches that in the endorsed_tcb field in the quote.

The 'correct' fix will probably be to add a new set of 'good' tcbs. This can then get populated with the current TCB on network creation and then updated via a governance action.