Parse CRS information using a custom decoder

[maxrjones]

I was originally thinking demonstrating loading and using COGs and/or GeoZarrs in Xarray with flexible coordinates and a xproj-backed CRS would require a custom backend but @TomNicholas pointed out that a clearer separation of concerns would be to implement a custom decoder (xref pydata/xarray#4490 (comment)) which would align to an API like ds = xr.open_dataset(store_path, engine='zarr', decoders=CRSDecoder).