update dtype example to use public API

[d-v-b]

Our custom dtype example relies on imports from zarr.core, we should adjust those imports from zarr.dtype or some other non-core location. If there's anything we can't get from outside zarr.core, we should make those exports public.

[chuckwondo]

Per Claude's search to see what can be modified to use "public" imports in examples/custom_dtype/custom_dtype.py:

Currently imported from	Can be imported from
zarr.core.dtype → ZDType	zarr.dtype
zarr.core.dtype → data_type_registry	zarr.dtype
zarr.core.dtype.common → DataTypeValidationError	zarr.dtype

Symbol	Lives in
JSON	zarr.core.common only
ZarrFormat	zarr.core.common only
DTypeConfig_V2	zarr.core.dtype.common only
DTypeJSON	zarr.core.dtype.common only
check_dtype_spec_v2	zarr.core.dtype.common only

How would you like to deal with the items in the 2nd table?

[d-v-b]

I think we should put all the general-purpose types in zarr.types for now, then later upstreaming them into zarr-metadata. the logic for that is that these types are just describing the JSON structure of zarr metadata.

The validator for the shape of the types (check_dtype_spec_v2) should be put in zarr.dtype