test framework: optimize key manager's memory usage

[unnawut]

🗒️ Description

Instead of loading the public and private keys for all 12 validators...

1. Loads only the public key by default which is already relatively very small
2. Loads the private key only when signing is needed, advance it to next epoch (slot) after signing, then serializes back to SSZ (this avoids holding the deserialized key with millions of FP objects that was taking many hundreds of MB per key)

🔗 Related Issues or PRs

Closes #458

✅ 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]

oooh much faster build job too