upstream basic metadata classes #3913

IMO it would be very valuable for the python community if we had a separate package narrowly scoped to simple typeddict & dataclass models of zarr metadata documents. This package would support any python application that has to work with zarr metadata. We would immediately use package this over in pydantic-zarr instead of re-defining the array metadata structure.

Instead of defining classes like this inside zarr-python, we would define them in the upstream zarr-metadata (name tentative) package

My initial proposal: imitate how @LDeakin did things in zarrs, with separate packages for the core metadata, and for extension metadata. I propose sticking these packages inside the zarr-python repo.

Counting ##3867 this is my second request that we move toward a monorepo :)