Implement get dataset default config name
Implement `get_dataset_default_config_name`.

Now that we support setting a configuration as default in `push_to_hub` (see #6500), we need a programmatically way to know in advance which is the default configuration. This will be used in the Space to convert script-datasets to Parquet: https://huggingface.co/spaces/albertvillanova/convert-dataset-to-parquet

Follow-up of:
- #6500

CC: @severo 