feat: add schedule, schedule-item and availability relationships to Driver model

Connects the Driver Ember Data model to the core-api scheduling system by
adding three hasMany relationships:

- schedules: polymorphic Schedule records where the driver is the subject
- schedule_items: individual shift records (ScheduleItem) where the driver
  is the assignee (assignee_type='driver', assignee_uuid=driver.id)
- availabilities: ScheduleAvailability records for time-off and preferred
  working hours

These relationships are required by:
1. The new driver/schedule panel component which renders a per-driver
   FullCalendar shift management view
2. The Intelligent Order Allocation Engine AllocationPayloadBuilder which
   queries active schedule_items to inject shift time_window constraints
   into the VROOM optimization payload

Refs: fleetbase/fleetops#214

Author: Ronald A Richardson

addon/models/driver.js

@@ -31,6 +31,11 @@ export default class DriverModel extends Model {
     @belongsTo('vendor', { async: true }) vendor;
     @hasMany('custom-field-value', { async: false }) custom_field_values;
 
+    /** @scheduling-relationships */
+    @hasMany('schedule', { async: true }) schedules;
+    @hasMany('schedule-item', { async: true }) schedule_items;
+    @hasMany('schedule-availability', { async: true }) availabilities;
+
     /** @attributes */
     @attr('string') name;
     @attr('string') phone;