[lib] Introduce superblock padding

Summary:
Part of ENG-3099

This diff implements modified PKCS#7 padding, which operates on block basis, instead of byte basis.
It is described in this Notion doc.

Depends on D7059

Test Plan: Added unit tests. Also tested using my CLI tool and hex editor / viewer (e.g. xxd tool). The CLI tool is also available in the Notion doc above

Reviewers: atul, marcin, tomek

Reviewed By: atul

Subscribers: anunay, ashoat, tomek

Differential Revision: https://phab.comm.dev/D7060