Shard parquet in `download_and_prepare`
Following https://github.com/huggingface/datasets/pull/4724 (needs to be merged first)

It's good practice to shard parquet files to enable parallelism with spark/dask/etc.

I added the `max_shard_size` parameter to `download_and_prepare` (default to 500MB for parquet, and None for arrow).

```python
from datasets import *

output_dir = "./output_dir"  # also supports "s3://..."
builder = load_dataset_builder("squad")
builder.download_and_prepare(output_dir, file_format="parquet", max_shard_size="5MB")
```

### Implementation details

The examples are written to a parquet file until `ParquetWriter._num_bytes > max_shard_size`. When this happens, a new writer is instantiated to start writing the next shard. At the end, all the shards are renamed to include the total number of shards in their names: `{builder.name}-{split}-{shard_id:05d}-of-{num_shards:05d}.parquet`

I also added the `MAX_SHARD_SIZE` config variable (default to 500MB)

TODO:
- [x] docstrings
- [x] docs
- [x] tests

cc @severo 