Adds an endpoint to the XenForo 2 API to grab a full list of staff members from the board.
Endpoint can be accessed at /api/users/staff with a valid XF-Api-Key.
Huge thanks to Sim on XenForo forums for his release of 'Find User by Criteria' for the XenForo API, which pointed me in the right direction with this. Recommend also installing his addon if you're starting to use the XenForo API: https://xenforo.com/community/resources/api-endpoint-find-a-user-by-criteria.7876/
(C) Chris Verringer, 2021