Add --connection-duration argument to specify the duration of MQTT connections

[bekicot]

Would it be possible to add a new command-line argument to the mqtt-benchmark tool, like:

mqtt-benchmark --connection-duration 10000

When specified, mqtt-benchmark would keep the connections open for all clients for the specified duration, in this case, 10 seconds. This would allow users to customize the connection duration for benchmarking purposes and better understand how the MQTT broker performs under different connection durations.