add optional secondsThreshold value

src/notifications/schedulers/ETANotificationScheduler.ts

@@ -7,6 +7,17 @@ export interface ScheduledNotificationData {
   deviceId: string;
   shuttleId: string;
   stopId: string;
+
+  /**
+   * Value which specifies the ETA of the shuttle for when
+   * the notification should fire.
+   * For example, a secondsThreshold of 180 would mean that the notification
+   * fires when the ETA drops below 3 minutes.
+   *
+   * Optional to retain backwards compatibility with devices on the
+   * older API version.
+   */
+  secondsThreshold?: number;
 }
 
 export class ETANotificationScheduler {