Request to add an option to store validation results in a file rather than on stdout

Some customers do not have access to BigQuery or choose not to use the service. When running DVT via a service like Cloud Run they have no way to easily capture the validation results.

We could add a file based alternative to `--bq-result-handler`, perhaps `--file-result-handler` or `--text-result-handler` that overrides writing to stdout with a file path. The file path should support cloud storage URIs.

We should give some thought to the option value format.

Is it as simple as a path string or should we revamp how we supply the format too by accepting a JSON value? For example:
```
--file-result-handler='{"path": "gs://some-uri", "format": "csv", "mode": "overwrite"}'
```

And revamp `--bq-result-handler` in a similar way (perhaps deprecating `--service-account` while we are at it):
```
--bq-result-handler='{"project": "my-project", "table": "my_dataset.results_table", "service-account": None}'
```