[services] Tests - Blob - Add Get

Summary:
Depends on D4274

Adding Get method to blob's tests.

Test Plan: For now, this method's not invoked. The Rust project should build normally.

Reviewers: palys-swm, varun

Reviewed By: palys-swm, varun

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4275