[Identity] Refactor get user_id and device_id

Summary:
This is useful for all authenticated requests, refactor.

Part of: https://linear.app/comm/issue/ENG-4445

Depends on D8605

Test Plan:

cd identity && cargo build

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8606