[lib] Introduce rolePermissionToBitmaskHex

Summary:
Introduce rolePermissionToBitmaskHex which turns threadRolePermission string into hex-encoded bitmask.

Context: https://linear.app/comm/issue/ENG-5388/encodepermissions-for-rolepermissionsblob

Test Plan: Unit tests, more testing in upcoming diff which handles decoding back to rolePermissionString

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9662