Serialize to_kerchunk using Zarr format 3

[maxrjones]

@mpiannucci pointed out that Kerchunk's reference specification is agnostic to the Zarr format. I think we should use Zarr format 3 rather than Zarr format 2 in the kerchunk writer to reduce the potential for bugs impacting the metadata conversation. This would also require updating the Kerchunk parser to parser Zarr format 3 references.

[TomNicholas]

This is probably a good idea. Questions:

1. Does the kerchunk backend for xarray support reading v3-format references?
2. Should we keep both? Selecting via a zarr_format kwarg that defaults to 3. Otherwise we would be making a breaking change with no way to opt-in to the old behaviour...