[lib] Introduce a special role for Admins

Summary:
We now introduce an enum that maps an ADMIN_ROLE to a number, similar to DEFAULT_ROLE

Addresses ENG-5835

Test Plan: N/A, tested in the next diff

Reviewers: atul, ginsu, tomek

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10075