[v3] declarative hierarchy API

[jhamman]

[d-v-b]

to elaborate: Zarr should support defining hierarchies of groups and arrays in a storage-independent manner. These hierarchy models can then be serialized to storage, which will create zarr metadata documents and return a handle to a normal zarr group or array, which can be manipulated normally. But the hierarchy models don't have to be serialized to storage immediately, or ever. See pydantic-zarr for an implementation of this idea.