Part of Part of [[ https://linear.app/comm/issue/ENG-3099/implement-pkcs7-padding | ENG-3099 ]]
This diff implements modified PKCS#7 padding, which operates on block basis, instead of byte basis.
It is described in [[ https://www.notion.so/commapp/Multimedia-encryption-protocol-88e8ca485930458cbbb6dae80d04e621?pvs=4#d891af4c73954f6faa02593253628b38 | this Notion doc ]].
Depends on D7059