Sharding in zarr v2

[d-v-b]

Since sharding is implemented as a codec, and because the zarr v2 spec takes a permissive stance to codecs, the sharding codec can be used for zarr v2 arrays, provided a program exists that can interpret that codec, such as any zarr v3 implementation. I can think of a variety of use cases for this (e.g., all use cases imagined for sharding in zarr v3), but the most simple one might be for people with v2 data who want sharding but don't want to change anything else about their workflow.

FWIW I haven't tested what the v3 branch of zarr-python would do with a v2 array with a sharding codec, but IMO failing to open it would be a bug.