When writing JSON output of an eval, include the timestamp of the run

[efung]

In the SQLite database of promptfoo, I can see that evals have a createdat timestamp, and this is visible in the Web UI as well.

However, when running via the CLI using something like npx promptfoo eval --output eval_output.json, the JSON file does not contain a timestamp anywhere.

It would be nice if a timestamp could be included somewhere in the output dictionary.

[typpo]

Done in #785 !

[typpo]

Released in 0.59.0