[services][blob] Add helpers and constants for Get RPC

Summary:

• Added helper convenience functions to get S3 path from database
• Copied constants from C++ service with comments

Depends on D5694

Test Plan: These functions are tested in next diff when they are used.

Reviewers: varun, jon, tomek, marcin

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5700