checkpoint sync API specification

[ch4r10t33r]

🗒️ Description

• Add HTTP API server for checkpoint sync, matching zeam's implementation
• New /lean/states/finalized endpoint serves finalized state as SSZ binary for fast node bootstrapping
• New /health endpoint for service health checks
• Add client utilities for fetching and verifying finalized state from remote nodes

🔗 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.

[tcoratger]

Just left comments about some simplification to reduce the amount of code but the logic looks reasonable to me.

[tcoratger]

Looks good to me, thanks a lot.