feat: Add user-accessible APIs for managing shard index caching

[kylebarron]

Change list

• Adds two new classes: ShardCache and AsyncShardCache. These map to the upstream ArrayShardedReadableExtCache and AsyncArrayShardedReadableExtCache, which are required arguments when reading sharded data.
• These classes hold the shard index. So if you read multiple shards from a single array, it should only have to read the shard metadata once.

Todo:

• We need to validate that these shard caches are tied to the same array. They can't be used across multiple different Array objects I don't think. We'll improve this validation when we implement array equality.

Closes #125

[kylebarron]

@d-v-b I'm merging this to make it easier to iterate; but happy to make changes if you have feedback!

[d-v-b]

+1 to moving fast! my two questions after a cursory read -- how are nested subchunks handled, and what's the contract between an arbitrary array-bytes codec and the array that allows the array to use the cache? I assume there's a trait in zarrs for "contains subchunks".