[comm-lib] Implement FromRequest for AuthService

Summary:
Extracted code that retrieves AuthService into a dedicated trait implementation.
Inspired by the web::Data counterpart.

Depends on D12504

Test Plan:
Local backup and reports services. Commenting out .app_data(auth_service) from http/mod causes
the error message to be printed.

Reviewers: kamil, varun, will

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12506