[services][backup] Add BackupItem database methods

Summary:
Implemented methods for managing BackupItems and the backup-service-backup table.

Depends on D6179

Test Plan: Called these locally and ensured that entities are properly inserted/read/removed from database.

Reviewers: varun, tomek, jon, max

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6180