[services] Backup - redesign proto

Summary:
Based on this revision: D2950
Adding proto file as discussed in the meeting and also as an introduction to implementing a new version of backup service.
This proto should be compatible with the API described in the linked revision.

Test Plan: grpc files generated successfully.

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

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3076