[backup-service] Rename 'PutClient' to 'BlobUploader'

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

Renamed PutClient to BlobUploader, 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 real Blob Client instance, introduced in further diffs.

Depends on D6382

Test Plan: Backup service compiles and starts.

Reviewers: varun, jon, michal, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6383