Custom metadata filenames

[lhoestq]

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.

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

[HuggingFaceDocBuilderDev]

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