Custom metadata filenames
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7663). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.
example: https://huggingface.co/datasets/lhoestq/overlapping-subsets-imagefolder/tree/main

To make multiple subsets for an imagefolder (one metadata file per subset), e.g.

```yaml
configs:
  - config_name: default
    metadata_filenames:
      - metadata.csv
  - config_name: other
    metadata_filenames:
      - metadata2.csv
```