Introduced additional way of file transfer interactions. Now client may send file transfer commands to `$file-async/...` topic instead of `$file/...` and receive command execution results as messages to `$file-response/{clientId}` topic.
This simplifies file transfer feature usage in certain cases, for example, when a client uses MQTTv3 or when the broker is behind an MQTT bridge.
See the [EIP-0021](https://github.com/emqx/eip) for more details.