Add missing parquet known extension
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{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/2733', 'html_url': 'https://github.com/huggingface/datasets/pull/2733', 'diff_url': 'https://github.com/huggingface/datasets/pull/2733.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2733.patch', 'merged_at': datetime.datetime(2021, 7, 30, 13, 24, 30)}
This code was failing because the parquet extension wasn't recognized:

```python
from datasets import load_dataset
base_url = "https://storage.googleapis.com/huggingface-nlp/cache/datasets/wikipedia/20200501.en/1.0.0/"
data_files = {"train": base_url + "wikipedia-train.parquet"}
wiki = load_dataset("parquet", data_files=data_files, split="train", streaming=True)
```

It raises
```python
NotImplementedError: Extraction protocol for file at https://storage.googleapis.com/huggingface-nlp/cache/datasets/wikipedia/20200501.en/1.0.0/wikipedia-train.parquet is not implemented yet
```

I added `parquet` to the list of known extensions

EDIT: added pickle, conllu, xml extensions as well
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