simplify store mappings

[anshalshukla]

🗒️ Description

• Refactors store to update signatures and payloads mappings
• Removes attestation_data_by_root mapping as with the modified mappings this is not required
• Simplifies the overall block building / pruning process
• These maps were originally designed when naive signature aggregation was performed, so each validator_id/ att_data was more convenient design, with leanVm the proposed design is more intuitive
• This should simplify the devnet-4 PR (can be treated as a phased PR for devnet-4 although this should not break devnet-3 compatibility)

🔗 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.