How to reset the client's timeout timer using MQTTnet?

To reset the client's timeout timer in MQTTnet, you can use the `MqttClient.PingAsync` method. This method sends a PINGREQ message to the broker, which causes the broker to respond with a PINGRESP message. When the client receives the PINGRESP message, it knows that the connection is still alive, and the timeout timer is reset.

Here's an example code snippet that demonstrates how to use the `PingAsync` method:

var options = new MqttClientOptionsBuilder()
    .WithTcpServer("broker.example.com", 1883)
    .WithClientId("myClient")
    .Build();

var factory = new MqttFactory();
var client = factory.CreateMqttClient();

await client.ConnectAsync(options);

// Send a PINGREQ message to reset the timeout timer
await client.PingAsync();

In this example, we create an instance of `MqttClientOptions` with the broker's address and port number, as well as a client ID. We then create an instance of `MqttClient` and connect to the broker using the `ConnectAsync` method. Finally, we call the `PingAsync` method to send a PINGREQ message and reset the timeout timer.

Note that the `PingAsync` method is asynchronous, so you need to use the `await` keyword to wait for the method to complete. Also, you should call the `PingAsync` method periodically to keep the connection alive and prevent the timeout timer from expiring.