[runner subcmd] add --raw-data flag to export raw data instead of aggregated one.

[fuweid]

Background

The multirunner server requires runner to upload all the latency values. So the runner run should support to export raw data if --raw-data flag set.

Plan

We need to refactor printResponseStats. When --raw-data is set, we can render data into json format, like

{
    total: INT,
    failures: [
       STRING,
       STRING,
       ...
    ],
    latencies: [
        FLOAT64,
        FLOAT64,
        ...
    ]
}