[services][blob] Add methods to manage the blob table

Summary:
Implemented methods for managing the blob-service-blob table. They will be needed in subsequent diffs.

This is mostly the same approach as in Identity Service, except I use anyhow to handle errors for now, for the sake of simplicity. Proper error handling is going to be introduced later.

Link to the C++ counterpart

Part of ENG-2300

Test Plan: Called these locally and ensured that entities are properly inserted/read/removed from database.

Reviewers: varun, jon, tomek, marcin

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5693