[identity] Add config for Backup service URL

Summary:
Add Backup service URL to Identity config, to let Identity call Backup endpoints

Depends on D12971

Test Plan: Cargo build. Played with clap CLI args in the Terminal

Reviewers: varun, will, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12972