[services] Backup - Send log - add generate holder

Summary:
Depends on D3648

Adding the generateHolder method - for now, it's mocked, but I think it's important to be aware that it should be implemented and this diff clearly indicates that.

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, geekbrother

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3649