fix: serialize signatures to fixed bytes

[unnawut]

🗒️ Description

This PR aligns leanSpec signature encoding (as SSZ container with subfields) with how the client implementations are doing (fixed bytes).

Because leanSpec has flags for different signature scheme (test and prod currently), I've updated the Signature container to calculate its length dynamically with TARGET_CONFIG.SIGNATURE_LEN_BYTES so it aligns with the --scheme flag used. However, in the cross client signature test, it's hardcoded to use PROD_CONFIG.SIGNATURE_LEN_BYTES for consistent test result.

The _empty_signature() function also has to be updated to reflect this change, but instead I replace it with consensus_testing.keys import create_dummy_signature() so there's one less place to maintain the logic.

This PR is a spec change but it aligns the specs with how the clients are doing, so no changes would be needed on the clients.

🔗 Related Issues or PRs

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.