Support async functions in map()
e.g. to download images or call an inference API like HF or vLLM

```python
import asyncio
import random

from datasets import Dataset


async def f(x):
    await asyncio.sleep(random.random())

ds = Dataset.from_dict({"data": range(100)})
ds.map(f)
# Map: 100%|█████████████████████████████| 100/100 [00:01<00:00, 99.81 examples/s]
```

TODO
- [x] clean code (right now it's a big copy paste)
- [x] batched
- [x] Dataset.map()
- [x] IterableDataset.map()
- [x] Dataset.filter()
- [x] IterableDataset.filter()
- [x] test
- [x] docs