add_column should preserve _indexes
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https://github.com/huggingface/datasets/issues/3769#issuecomment-1167146126

doing `.add_column("x",x_data)` also removed any `_indexes` on the dataset, decided this shouldn't be the case.

This was because `add_column` was creating a new `Dataset(...)` and wasn't possible to pass indexes on init.
with this PR now can pass 'indexes' on init through `IndexableMixin`

- [x] Added test