Delivery receipt can be used to see whether a message has been successfully delivered to all intended recipients by the Sendbird server. To mark messages as delivered when a group channel member successfully receives a push notification for the message from APNs, the
markAsDelivered(remoteNotificationPayload:completionHandler:) method should be implemented in
Notification Service Extension.
When a message is delivered to a group channel member who is online, the message is automatically marked as delivered and the other members who are online are notified of the delivery receipt through the
channelDidUpdateDeliveryStatus(_:) method of