[services-lib] Reorganize auth module

Summary:
Just a reorganization of the auth module. No functional changes. auth.rs -> auth/mod.rs + auth/types.rs

Depends on D9276

Test Plan: Cargo check

Reviewers: michal, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9277