[backup] Hotfix Blob service client auth

Summary:
Noticed that Backup service had code respoinsible for extracting UserIdentity (auth metadata),
but it was never supplied to BlobServiceClient so Backup -> Blob service requests were unauthenticated.

This is not the nicest solution but it works. A proper solution will be introduced in the next diffs.

Test Plan: Backup service on staging

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12504