Re-implement support for parsing non-listable zarr stores

[TomNicholas]

#892 assumes we can list the bucket.

See discussion on #919 - this issue can be closed once the test added in #919 can be un-xfailed.

[TomNicholas]

Wait actually @norlandrhagen @maxrjones it seems the original implementation of the ZarrParser actually used list_prefix to find the populated keys. It still used .getsize on every one of those keys to find the object size, so the new parser is still a big improvement. But this means that #892 was not a regression - we never supported non-listable stores!

That makes me think we should not consider this a blocker to releasing, and only bother implementing it when someone actually wants it.

[norlandrhagen]

Great catch @TomNicholas! ➕