Use simple list of mock users for sandbox

[junminahn]

Description
We are transitioning from using m364-proxy server to m365-mock. As part of this change, we will deprecate the proxy server. During this process, the user list in /localdev/m365proxy/mocks.json should be refactored into a flat structure and relocated to a more generic directory for better organization.

Technical Details

• Refactor the user list into a flat structure and move it to a common/generic directory.
• Update any related application and sandbox server configurations that depend on this file.

Definition of Done (DOD)

• The application's tests and sandbox environment function as expected without any issues or regressions.