[services] Backup - Move Service Blob Client

Summary:
Depends on D3612

I think we should separate reactors' code and clients' code(there may be more clients in the future).

Test Plan:

cd services
yarn run-backup-service

Reviewers: geekbrother, varun, jimpo, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3532