feat(common): encode the SNARK aggregate signature as bytes in the certificate message

[jpraynaud]

Content

This PR includes the encoding of the certificate message aggregate signature by type, so SNARK and IVC proofs use the compact bytes-hex encoding while concatenation proofs remain backward compatible:

• Encode the certificate message multi_signature field by aggregate signature type: JSON-hex for concatenation proofs (backward compatible with all deployed clients) and bytes-hex for SNARK and IVC proofs.
• Route the genesis signatures through the native ProtocolKey codec (String::try_from) instead of an explicit bytes-hex call.
• Keep the certificate hash and chain walking JSON-hex based, so certificate hashes stay byte-identical.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

Issue(s)

Closes #3379

[github-actions]

Test Results

     5 files  ± 0     209 suites  ±0   59m 13s ⏱️ - 56m 13s
 3 322 tests  - 51   3 322 ✅  - 51  0 💤 ±0  0 ❌ ±0 
11 055 runs   - 53  11 055 ✅  - 53  0 💤 ±0  0 ❌ ±0 

Results for commit 6981ddd. ± Comparison against base commit 8bbdf2d.