fix: populate common_name in CertificateInfo for CAWG identity assertions

[rajat-2023]

Problem

cawg.identity.signature_info.common_name is always absent when verifying
a CAWG identity assertion, even when the signing certificate has a valid CN.
The top-level manifest signature.common_name (standard C2PA) works correctly
because it uses a different code path.

Root Cause

Two divergent code paths exist in c2pa:

• src/cose_validator.rs → get_signing_info() (standard C2PA manifest signatures)
  calls extract_common_name_from_cert() and sets common_name in CertificateInfo. ✅

• src/crypto/cose/verifier.rs → verify_signature() (CAWG identity assertions)
  builds CertificateInfo with ..Default::default(), leaving common_name: None. ❌

The CertificateInfo struct (certificate_info.rs:38) has pub common_name: Option<String>
defined, and cose_validator.rs correctly populates it. But when verifier.rs was
introduced in #801 (Dec 2024) to handle the CAWG identity verification path,
common_name was never ported over.

Fix

Extract common_name from the end-entity cert's subject in verifier.rs,
using the same x509_parser approach already used for issuer_org just above.

Testing

Verified by:

1. Signing a JPEG with a CAWG identity assertion (cert CN = "rajat customer")
2. Before fix: c2patool file.jpeg -d shows cawg.identity.signature_info
   with alg, issuer, cert_serial_number but no common_name
3. After fix: common_name: "rajat customer" appears correctly