[keyserver] Create a function that downloads blobs

Summary:
This function takes a hash as a parameter and returns a blob.

Depends on D9941

Test Plan: Create a script that creates a blob and then downloads it - check if the right payload is received.

Reviewers: kamil, marcin, bartek

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9942