[services-lib] Add enum for authorization tokens
Summary:
Part of ENG-4939. We need to support both access token coming from users (commServicesAccessToken) and private service-to-service token. I've decided to create an enum for this
NOTE: I'm open to naming suggestions
Test Plan: Added unit tests. Also tested later in the stack
Reviewers: michal, varun, jon
Reviewed By: michal
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D9241