[services] Backup - Add client reactor base classes - bidi reactor

Summary:
Depends on D3513

Adding client base classes for reactors - bidi reactor

Test Plan:

cd services
yarn run-backup-service

Reviewers: geekbrother, jimpo, varun, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3465