tests: add test vectors for SSZ containers

[unnawut]

🗒️ Description

Add test vectors for SSZ containers

✅ 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.

[unnawut]

Need Fp implementation here so that Fp in the Signature can be serialized and merkleized

[tcoratger]

Thanks a lot!