Load raster lazy coordinates

[benbovy]

As discussed with @snowman2, there may be cases where it is preferable to have raster coordinates with pre-computed values instead of lazy values (e.g., avoid re-computing values each time when repeatedly indexing a relatively small raster dataset).

Reusing Xarray's .load() or .compute() API would be nice. We'd need to extend Xarray's Index API with a similar method that may be overridden pydata/xarray#8128.