[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.

[d-v-b]

cc @ianhi, your pr #3914 reminds me that this issue is still open. clearly there's value in a model of a hierarchy that doesn't reference a concrete store implementation. we should use the issue to sketch out what we want that model to do.