[native] Introduce StaffContext

Summary:
Context: https://linear.app/comm/issue/ENG-2134/have-a-flag-that-indicates-whether-the-build-is-a-staff-release

This diff is essentially copy/paste of native/root-context.js (which is why I made it a separate diff)

Depends on D5498

Test Plan: NA

Reviewers: marcin, tomek, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5499