Function verify_seal 

pub fn verify_seal(
    manifest_bytes: &[u8],
    seal_bytes: &[u8],
) -> Result<SealVerification>

Verify a CMS seal against the manifest bytes.

Checks:

1. Parses as valid CMS SignedData
2. The message-digest signed attribute matches SHA-256(manifest_bytes)
3. The signature over the signed attributes is valid (ECDSA P-256)