[services-lib] Introduce AuthService for token verification

Summary:
Created a helper "service struct" to easily handle token operations. It allows to obtain a service-to-service token and to verify a token's validity.
For UserIdentity (client tokens) it calls Identity Service to verify the token.

This struct is designed to be provided as app-data to HTTP actix servers - it's going to be used by auth middleware.

Depends on D9279

Test Plan: cargo check, logic is tested together with next diffs

Reviewers: varun, michal, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9280