Machine-readable output of `btrbk list`

I have a use-case where I need to determine the latest btrbk snapshot for some subvol.

I had to resort to parsing the raw format using a regex:

```sh
btrbk list --format=raw | rg "^.* snapshot_subvolume='(/.*?/snapshotnamehere\..*?)' .*$" --replace '$1' | tail -n 1
```

This would have been much simpler (not to mention more robust) if there was an option to output a standardised machine-readable format such as JSON.

Start at the `btrbk list` command and inspect the existing `--format=raw` output, including the snapshot fields the issue's regex extracts. Decide on a standardized machine-readable schema and define what the new output option must emit for relevant subvolumes and snapshots. Done means callers can reliably consume the output without regex parsing.

Written by the indexing model from the issue text.