fix(polka-storage-proofs): add sector padding

[th7nder]

Description

Fixes #420.
Sector file is always full-sized one, no matter how many pieces are inside.
To generate a valid CommD from pieces, so a proof can be generated, we need to pad the sector with 'dummy' pieces so the CommD calculation from pieces is the same as the one from raw disk data.

This PR allows to create a Sector via create_sector function from pieces which do not take the entire space of the sector.

Important points for reviewers

When implementing #360 (or merging #423), the interface will probably need to be changed a bit so it accomodates use cases outside the PoRep Demo.
I didn't want to make it now, cause i don't know the shape of a client yet.

Checklist

• Are there important points that reviewers should know?
  • If yes, which ones?
• Make sure that you described what this change does.
• If there are follow-ups, have you created issues for them?
  • If yes, which ones? / List them here
• Have you tested this solution?
• Were there any alternative implementations considered?
• Did you document new (or modified) APIs?

[jmg-duarte]

Some questions, overall looks good. Nice job 🚀

[cernicc]

Nice 👍