Allow specifying default chunks per shard in `to_zarr`. The `zarr_chunks_per_shard` argument in `xbeam.Dataset.to_zarr` now supports using `...` as a key to set a default number of chunks per shard for all dimensions not explicitly listed. Dimensions not included in the mapping default to 1 chunk per shard. This simplifies specifying Zarr chunking strategies. PiperOrigin-RevId: 819301545