[services][backup] Add LogItem database methods

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

Depends on D6180

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

Reviewers: varun, tomek, jon, max

Reviewed By: varun

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6181