Enable streaming for Wikipedia corpora
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**Is your feature request related to a problem? Please describe.**
Several of the [Wikipedia corpora](https://huggingface.co/datasets?search=wiki) on the Hub involve quite large files that would be a good candidate for streaming. Currently it is not possible to stream these corpora:

```python
from datasets import load_dataset

# Throws ValueError: Builder wikipedia is not streamable.
wiki_dataset_streamed = load_dataset("wikipedia", "20200501.en", split="train", streaming=True)
```

Given that these corpora are derived from Wikipedia dumps in XML format which are then processed with Apache Beam, I am not sure whether streaming is possible in principle. The goal of this issue is to discuss whether this feature even makes sense :)

**Describe the solution you'd like**
It would be nice to be able to stream Wikipedia corpora from the Hub with something like

```python
from datasets import load_dataset

wiki_dataset_streamed = load_dataset("wikipedia", "20200501.en", split="train", streaming=True)
```
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