[services] Backup - Add random string to tools

Summary:
Depends on D3462

I needed a random string generator for testing the backup service. Once we're done mocking things in this service, this can be removed.

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm, jimpo, geekbrother, varun

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3463