feat: Implement submit_windowed_post extrinsic

[aidan46]

Description

Implement the submit_windowed_post extrinsic, allowing an SP to submit their Proof-of-Spacetime. The validation of the proof is stubbed out. I am not sure if always returning true is the way to go for now or having some type of failure condition is preferred, I am open to suggestions.

Important points for reviewers

Filecoin implementation reference

Checklist

• Are there important points that reviewers should know?
  • If yes, which ones?
    Proofs are not validated at the moment so the validate_windowed_post always returns true on the submitted proof.
• If there are follow-ups, have you created issues for them?
  • If yes, which ones? / List them here
    SP pallet (hooks): pre-commit hook #103 will check that the Proof-of-Spacetime is submitted on time.
• Have you tested this solution?
  I did not write a test for this as it always returns true for every proof
• Did you document new (or modified) APIs?

[jmg-duarte]

Preliminary questions and notes

[cernicc]

good work 💪

[jmg-duarte]

Let's merge this and do any leftover improvements as a result of further development. Good job!

Stale