iperf3 json model

This is a simple model for the iperf3 json output. It can be used to parse the json output of iperf3.

iperf3 -J -c <server>
...

    let json: &str = ...;
    let iperf = serde_json::from_str::<iperf3_json::Iperf3>(json).expect("failed to parse");