extratags=1 should return admin_level for boundary=administrative

The `admin_level` tag is currently saved in an extra column in the placex table and therefore not returned when requesting the full set of tags. That's rather inconvenient as the admin_level can be relevant for further processing, see for example https://github.com/openstreetmap/openstreetmap-website/issues/6870. We should just mix in the admin_level into the returned extratags for boundary=administrative (and only for that), when `extratags=1` is requested. This should be done on the [formatter level](https://nominatim.org/release-docs/develop/customize/Result-Formatting/). Check out the default formatters at `src/nominatim_api/v1/format*.py`. _Note: make sure to read the [contribution guide lines on AI](https://github.com/osm-search/Nominatim/blob/master/CONTRIBUTING.md#using-ai-assisted-code-generators) before starting to work on this._