Endpoint to list frontend users

[olliestanley]

• GET /frontend_users/: list users ordered by username, max_count (int, limit), ge (str, optional, >= comparison on username), lt (str, optional, < comparison on username), auth_method (str, optional)

[Vechtomov]

I need to clarify a few questions:

1. What are the bounds of max_count?
2. What fields should endpoint return?
3. Do we need to pass api_client_id to the endpoint as it was done in #539?