[services] Backup - Database - add Backup Item

Summary:
C++ representation of the backup database entity

Depends on D3084

Test Plan: -

Reviewers: palys-swm, geekbrother, varun, ashoat, jimpo

Reviewed By: palys-swm, ashoat

Subscribers: benschac, ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D3087