abstracting over zarr versions

[d-v-b]

operations like flattening a zarr hierarchy, operationally turning a v2.GroupSpec or v3.GroupSpec into dict[str, v2.GroupSpec | v2.ArraySpec] or dict[str, v3.GroupSpec | v3.ArraySpec]` have the same logic for zarr v2 and zarr v3. So we should be able to define the flattening operation in 1 place, and make it generic over the input / output types. Right now in #67 we are decidedly not doing this, and instead doing rank code duplication. This should be fixed and the logic consolidated as much as possible.