use zarr-cm for types?

[d-v-b]

zarr-cm defines the core types for the geozarr conventions, and defines some validation routines. The types are definitely re-usable here, and the validation routines are as well. It would probably be good to centralize on a single source of truth for these types across the python ecosystem. zarr-cm deliberately has minimal dependencies (just typing-extensions) to support usage in libraries like the geozarr-toolkit. would it be in-scope to de-duplicate functionality here by pulling in zarr-cm as a dependency?

[maxrjones]

Yes, great idea! A PR for this would be very welcome