[blob][reports] Provide identity service endpoint

Summary:
Provide identity service endpoint along with AuthService instance for blob and reports service. This is needed for service-to-service token auth and client verification.
Haven't touched backup service yet but it's subject to change as well (after client work is done).

Depends on D9280

Test Plan: Cargo check + ensured that CLI arg is read correctly in blob service

Reviewers: michal, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9281