[backup-service] Add blob client init function

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

Created a function that initializes a blob client instance. The connection is lazy-initialized so it won't be established until first use.

Depends on D6383

Test Plan: Backup service compiles.

Reviewers: varun, jon, michal, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6384