[services] Backup - Blob client reactors - add getStatus

Summary:
Depends on D3719

We need to be able to pull the status by external objects from blob client reactors.

Test Plan:

cd services
yarn run-backup-service

Reviewers: palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3720