[backup-service] Use shared 'BlobClient' in workers

Summary:
Part of https://linear.app/comm/issue/ENG-2688/backup-service-reuse-single-blob-client-instance

Modified BlobDownloader::start and BlobUploader::start to take BlobClient as an argument instead of creating its own instance.

Depends on D6385

Test Plan: Ran backup service integration and performance tests to confirm it works as expected

Reviewers: varun, jon, michal, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6386