Add Json() type

[lhoestq]

Json() type is needed when some fields don't have fixed types, e.g. "tools" and "tool_calls" types in conversation + tool calling datasets like dataclaw datasets. Cc @peteromallet and @WoctorDho @Nanbeige for viz

Examples of supported tool-calling / dataclaw datasets:

ds = load_dataset("peteromallet/dataclaw-peteromallet")  #  happens to not need Json() since tool types are fixed
ds = load_dataset("woctordho/dataclaw")
ds = load_dataset("Nanbeige/ToolMind")

The Json() type is auto-applied when loading JSON files when mixed types are found. The Json()type is set to the list containing objects with mixed types, to end up with a List(Json()) type for "tools" and "tool_calls" columns.

It is also possible to define a Dataset with a Json() type like this:

>>> from datasets import load_dataset, Dataset, Features, Json, List
>>> example = {"a": [{"key": 0}, {"another-key": "another-type"}]}
>>> features = Features({"a": List(Json())})
>>> ds = Dataset.from_list([example], features=features)
>>> ds[0]
{'a': [{'key': 0}, {'another-key': 'another-type'}]}

close #7869
close #4120
close #5827
close #7418
related to #7092
related to #6162
related to #2799
related to https://huggingface.co/datasets/PatoFlamejanteTV/LocalLLaMA/discussions/1

[HuggingFaceDocBuilderDev]

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