API for accessing COG overviews

agreed, 

I just need a way to do 
```python
await with GeoTIFF.open("geo.tif") as dataset:
    overview = dataset.get_overview(1)
    ovr_width, ovr_height = overview.width, overview.height
    ovr_blocksize = overview.blocksize
```

_Originally posted by @vincentsarago in https://github.com/developmentseed/async-geotiff/pull/2#discussion_r2729366143_
            