Dataclass for "VirtualZarrStore"

Problem

Kerchunk user code currently passes around an obscure multiply-nested "reference dict" object. This is hard to read, interrogate, validate, or reason about.

Suggestion

Instead create a new VirtualZarrStore dataclass, which contains all the same information that is currently stored in the reference dict but in a more structured manner. This would then be the principle object that gets passed around between user calls to kerchunk API.

Advantages

• Easier to read and interrogate than multiply-nested dicts
• Allows direct validation
• Serializes in obvious ways (via .to_json, to_parquet, .to_dict or similar.)
• Easier to write tests, by using fixtures to generate VirtualZarrStore objects
• Concentrates concerns over changes/enhancements to Zarr Spec in one class
• A v2->v3 converter could act directly on these objects
• Possibly easier to understand whenever anyone reimplements kerchunk in other languages?

Implementation ideas

• Implementation could subclass Zarr Object Model classes (where .to_json is analogous to the ZOM's .serialize), which then would be solidified as the recommended abstract representation once ZEP006 is accepted
• Can't use a bare ZOM class because we need to add some extra attributes for byte ranges etc. However information on where to find chunks is essentially a "Chunk Manifest", a generalizable idea that @jhamman has also been working on (for a nascent ZEP007??)
• Attributes of this dataclass need to always be serializable, so the VirtualZarrStore should be basically a json schema (see #373)

Questions

• Is it possible to do this in a broadly backwards-compatible manner?

Begin by reviewing how kerchunk currently passes and serializes its nested reference dictionaries, then read the linked Zarr Object Model and ZEP006 discussions. Define the scope and backwards-compatibility requirements for a VirtualZarrStore before implementation; done would require an agreed structured representation, serialization approach, and migration path.