Message status

Notification of the status of an outbound message (`type: status`). A single message can generate several events in order: `SENT`, `DELIVERED`, `READ`. The `FAILED` status is terminal (no more events are received after it) and carries Meta's `errors` array with the failure detail. Not all messages reach `READ`; it depends on whether the user has read receipts enabled. The order may vary: in some cases `DELIVERED` and `READ` are received almost simultaneously.