Keep features after transform
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["One note on features inference:\r\n\r\nif an arrow type is `struct of items` where each item is a `list`, then we return a `dict` in which each item is a `Sequence`.\r\nIt means that we don't use the Sequence <-> dict swap when we infer features.\r\n\r\nIt's fine because the swap is generally used in dataset scripts, in which features are defined (inferred features are discarded)"
 "If it's fine for you @thomwolf we can merge this one :) "
 'Yes this is fine I think!']
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When applying a transform like `map`, some features were lost (and inferred features were used).
It was the case for ClassLabel, Translation, etc.

To fix that, I did some modifications in the `ArrowWriter`:

- added the `update_features` parameter. When it's `True`, then the features specified by the user (if any) can be updated with inferred features if their type don't match. `map` transform sets `update_features=True` when writing to cache file or buffer. Features won't change by default in `map`.

- added the `with_metadata` parameter. If `True`, the `features` (after update) will be written inside the metadata of the schema in this format:
```
{
    "huggingface": {"features" : <serialized Features exactly like dataset_info.json>}
} 
```
Then, once a dataset is instantiated without info/features, these metadata are used to set the features of the dataset.
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