Function validate_ed25519_cert 

fn validate_ed25519_cert(
    cert: ParsedEd25519Cert,
    subject: &PublicKey,
    signed_with: &PublicKey,
    cert_type: CertType,
    ts: &SystemTime,
) -> Result<EncodedEd25519Cert, InvalidCertError>

Validate the specified cert, checking that

• its CertType is `cert_type, and
• its subject key is subject, and
• it is signed with the signed_with key, and
• it is timely (it is not expired or not yet valid at the specified ts)