Support streaming in size estimation function in `push_to_hub`
would this allow to include the size in the dataset info without downloading the files? related to https://github.com/huggingface/datasets/pull/3670 @severo I don't think so. We could use this to get `info.download_checksums["num_bytes"]`, but we must process the files to get the rest of the size info. 
This PR adds the streamable version of `os.path.getsize` (`fsspec` can return `None`, so we fall back to `fs.open` to make it more robust) to account for possible streamable paths in the nested `extra_nbytes_visitor` function inside `push_to_hub`.