Local paths in common voice
I just changed to `dl_manager.is_streaming` rather than an additional parameter `streaming` that has to be handled by the DatasetBuilder class - this way the streaming logic doesn't interfere with the base builder's code.

I think it's better this way, but let me know if you preferred the previous way and I can revert

> But on the other hand, IMHO, I think this specific solution adds complexity to handling streaming/non-streaming, and moves this complexity to the loading script and thus to the contributors/users who want to create the loading script for their canonical/community datasets (instead of keeping it hidden form the end users).

I'm down to discuss this more in the future ! @lhoestq good idea: much cleaner this way! That way each class has its own responsibilities without mixing around...
Continuation of https://github.com/huggingface/datasets/pull/3664:

- pass the `streaming` parameter to _split_generator
- update @anton-l's code to use this parameter for `common_voice`
- add a comment to explain why we use `download_and_extract` in non-streaming and `iter_archive` in streaming

Now the `common_voice` dataset has a local path back in `ds["path"]`, and this field is `None` in streaming mode.

cc @patrickvonplaten @anton-l @albertvillanova 

Fix #3663.