[services][backup] Implement blob Get client

Summary:
This diff implements the blob Get client logic. Added extensive in-code comments describing its working principle.

This code is an improved and simplified version of old blob_client: (link) - I got rid of custom Tokio runtimes and mutexes, which caused problems in the previous implementation.

Depends on D6165

Test Plan: This code is tested in subsequent diffs where it is used in backup service ednpoints.

Reviewers: varun, tomek, jon, max

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6166