Support structured logging

Hello!

We run `pint` as a daemonset alongside our Prometheus statefulsets and while it's largely great & very helpful, GCP doesn't parse the logs at the right level since they're output to `stderr` and are just text logs:

Would you consider adding support for structured logging? I'd be happy to try a PR to add support via a flag, config file setting, or both. It looks like `slog` has a JSONHandler [type](https://pkg.go.dev/log/slog#JSONHandler), so I imagine it wouldn't be too difficult.

Cheers!