More robust `None` handling
PyArrow has explicit support for `null` values, so it makes sense to support Nones on our side as well.

[Colab Notebook with examples](https://colab.research.google.com/drive/1zcK8BnZYnRe3Ao2271u1T19ag9zLEiy3?usp=sharing)

Changes:
* allow None for the features types with special encoding (`ClassLabel, TranslationVariableLanguages, Value, _ArrayXD`)
* handle None in `class_encode_column` (also there is an option to stringify Nones and treat them as a class)
* support None sorting in `sort` (use pandas for that) 
* handle None in align_labels_with_mapping
* support for None in ArrayXD (converts `None` to `np.nan` to align the behavior with PyArrow)
* support for None in the Audio/Image feature
* allow promotion when concatenating tables (`pa.concat_tables(table_list, promote=True)`) and `null` row/~~column~~ broadcasting similar to pandas 

Additional notes:
* use `null` instead of `none` for function arguments for consistency with existing `disable_nullable` 
* fixes a bug with the `update_metadata_with_features` call in `Dataset.rename_columns`
* had to update some tests, let me know if that's ok

TODO:
- [x] check how the Audio features behaves with Nones
- [x] Better None handling in `concatenate_datasets`/`add_item`
- [x] Fix formatting with Nones
- [x] Add Colab with examples
- [x] Tests

TODOs for subsequent PRs:
-  Mention None handling in the docs
- Add `drop_null`/`fill_null` to `Dataset`/`DatasetDict`

Fix #3181 #3253