[native-rust-library] Backup service config

Summary:
ENG-5346

This diff introduces the config for backup URL and refactors the existing identity service config code so that it can be reused between identity and backup.

Test Plan:
Added assert_eq!(BACKUP_SOCKET_ADDR, ""); in the test to get the current value of BACKUP_SOCKET_ADDR in the panic message. Then I've run these command and checked the value of BACKUP_SOCKET_ADDR:

• cargo test --release, no file -> backup.commtechnologies.org
• cargo test --release, file -> value in the file
• cargo test, file -> value in the file
• cargo test, no file -> backup.staging.commtechnologies.org

Reviewers: varun, bartek, kamil

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10261