Fix NQ features loading: reorder fields of features to match nested fields order in arrow data

[lhoestq]

As mentioned in #2401, there is an issue when loading the features of natural_questions since the order of the nested fields in the features don't match. The order is important since it matters for the underlying arrow schema.

To fix that I re-order the features based on the arrow schema:

inferred_features = Features.from_arrow_schema(arrow_table.schema)
self.info.features = self.info.features.reorder_fields_as(inferred_features)
assert self.info.features.type == inferred_features.type

The re-ordering is a recursive function. It takes into account that the Sequence feature type is a struct of list and not a list of struct.

Now it's possible to load natural_questions again :)