[lib] Introduce PKCS#7 padding

Summary:
Part of ENG-3099

This diff implements standard PKCS#7 padding using JS typed arrays.

Test Plan: Added unit tests. Also tested using my CLI tool and hex editor / viewer (e.g. xxd tool)

Reviewers: atul, marcin, tomek

Reviewed By: atul

Subscribers: anunay, ashoat, tomek

Differential Revision: https://phab.comm.dev/D7059