[services] Move reserved_usernames to shared

Summary:
Move the file to shared and import it in two places.

This solution still requires the file to be duplicated. Solving this is more involved, as it would require storing this file in json format and importing it in lib, which currently isn't supported. Even if we use fs instead, we would still need to change the logic to be async.

https://linear.app/comm/issue/ENG-6780/share-reserved-usernames-between-services

Depends on D11015

Test Plan: Run both Identity and Blob services on local and Identity on Docker and check if in each case the file is accessible by console logging its content.

Reviewers: michal, bartek, varun

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11051