Add columns types to /rows response

[severo]

We currently just have the keys (column identifier) and values. We might want to give each column's type: "our own serialization scheme".
For ClassLabel, this means to pass the "pretty names" (the names associates to the values) along with the type

See https://github.com/huggingface/moon-landing/pull/1040#discussion_r709496849

[severo]

Proposal: add a "features" key to the /rows response which will list the features for every config. Every element in the list will give the dataset, config and features of the config (see features in https://datasets-preview.huggingface.tech/info?dataset=acronym_identification&config=default):

{
  "dataset": "acronym_identification",
  "config": "default",
  "features": {
    "id": {
        "dtype": "string",
        "id": null,
        "_type": "Value"
    },
    "tokens": {
        "feature": {
            "dtype": "string",
            "id": null,
            "_type": "Value"
        },
        "length": -1,
        "id": null,
        "_type": "Sequence"
    },
    "labels": {
        "feature": {
            "num_classes": 5,
            "names": [
                "B-long",
                "B-short",
                "I-long",
                "I-short",
                "O"
            ],
            "names_file": null,
            "id": null,
            "_type": "ClassLabel"
        },
        "length": -1,
        "id": null,
        "_type": "Sequence"
    }
  }
}