add evaluation parameter metadata to output file

[wyatt-avilla]

This should be another JSON object at the top of the output file, containing metadata about the evaluation parameters and other helpful info (e.g, the datetime when the evaluation took place).

For example:

{
    "evaluation_type": "single-turn",
    "threshold": 0.7,
    "test_case_groups": ["group1"]
}

If we support a metadata object like this, it would make sense to simplify the default output file names.