Add open_dataset_kwargs

[TomAugspurger]

Currently, there's some ambiguity around whether users should use xr.open_dataset or xr.open_zarr.

Recent versions of xarray include an engine keyword in xr.open_dataset. So we can remove that ambiguity by adding a field like

        "xarray:open_dataset_kwargs": {
            "engine": "zarr",
            "backend_kwargs": {
                "consolidated": False,
                "chunks": {"time": 365},
            }
        }