[backup] Add AuthService to HTTP server

Summary:
We already have this for Blob and Reports. Backup needs this also for middleware to be working.
AuthService is a struct living in Actix-web data, that handles calling Identity for verifying CSAT,
and provides utilities for service-to-service tokens.

Test Plan: Tested in the next diff.

Reviewers: varun, will, marcin

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12413