[services] Backup - Implementation structure

Summary:
Implementation structure based on the .proto file

Depends on D3087

Test Plan: Backup service builds successfully

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/D3158