[services] Tests - Blob - Add blob utils

Summary:
Depends on D4272

Adding blob utils containing some stuff that will be used across all blob tests. BlobData is a helper structure for keeping the data that describes the blob record.

Test Plan: None really. Itself, it does not do anything special. Will be used later in the stack.

Reviewers: palys-swm, varun

Reviewed By: palys-swm, varun

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

Differential Revision: https://phab.comm.dev/D4273