[lib] Extract function to download blob

Summary:
Preparation for ENG-9526.
Replaced direct calls of fetch() with a dedicated function. It makes it easier to handle specific response code for invalid CSAT.

Test Plan: Flow, eslint, manual testing

Reviewers: kamil, varun, ashoat

Reviewed By: varun, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13958