[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