Make attrs JSON serializable

[TomAugspurger]

xstac will insert the attrs into each variable into the STAC item / collection. These attrs are not always JSON serializable, causing validation to fail.

The workaround is for users to manually call

    for k, v in ds.variables.items():
        attrs = {name: xr.backends.zarr.encode_zarr_attr_value(value) for name, value in v.attrs.items()}
        ds[k].attrs = attrs

we could do this for them.