[backup-service] Rename 'GetClient' to 'BlobDownloader'

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

Renamed GetClient to BlobDownloader, because this name is more accurate - these instances are not clients, but rather single RPC call instances. Also, old name could easily be confused with actual Blob Client instance, introduced in further diffs.

Test Plan: Backup service compiles and starts.

Reviewers: varun, jon, michal, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6382