Add writer_batch_size for ArrowBasedBuilder
This way we can control the size of the record_batches/row_groups of arrow/parquet files.

This can be useful for `datasets-server` to keep control of the row groups size which can affect random access performance for audio/image/video datasets

Right now having 1,000 examples per row group cause some image datasets to be pretty slow for random access (e.g. 4seconds for `beans` to get 20 rows)