GET maintenance/work-orders: Add work order assignees

The GET maintenance/work-orders endpoint has some new features! Added an optional assignees array to the work order response listing every dashboard user assigned to the work order. Each entry contains:

• userId - Samsara dashboard user ID of the assigned technician, joinable against the users API
  The existing assignedUserId field is unchanged and continues to return the primary assignee, which is also present in this array. The array is only returned for organizations with multiple work order assignees enabled.