Zarr PackBits filter

I see in the documentation that [Zarr filters are not yet supported](https://google.github.io/tensorstore/driver/zarr/index.html#json-driver/zarr.metadata.filters), so I will be working around this now but I just wanted to document my use case for this feature: storing masks for missingness in genotype data. [Sgkit](https://github.com/pystatgen/sgkit) generates Zarrs as their core data structure using the [PackBits](https://numcodecs.readthedocs.io/en/stable/packbits.html) filter for masks, so I hope filters will soon be supported.