Support compressed JSON duration files

would allow not polluting the line diff count in PRs that only update the durations file. ideally, users would only need to change the `--durations-path` extension to make this work:

```sh
pytest --store-durations --durations-path .pytest-split-durations.json.gz # file should now be gzip compressed
```

Start from the pytest command entry point and trace how --durations-path is read and how duration data is stored. Check the existing duration-file behavior, then verify that a .json.gz path is handled as gzip-compressed while ordinary duration paths continue to work.

Vom Indexierungsmodell aus dem Issue-Text verfasst.