Inject attestation bundle properties for policy validation

[jiparis]

ATTESTATION material type injects the In-toto statement to the policy engine. It would be good if, in addition to the statement:

• it verifies the attestation bundle (in Sigstore bundle format), including the certificate chain, TSA and signature
• it passes the signing certificate properties to the policy engine
• it passes the result of the verification to the policy engine

[jiparis]

Closing as policy evaluations are evaluated BEFORE signing.