Implement a Zarr Python adapter for FSMap inputs

[maxrjones]

Zarr Python 2.x accepted FSMap for a store input, whereas Zarr Python 3.x currently does not. We could either modify our code to explicitly pass a fsspec FileSystem, contribute an upstream convenience feature, or both. I would suggest both since a convenience layer would help a lot of people and it's also better to be explicit about using async file systems.