[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