Consider merging with `odc-stac`

`odc-stac` uses a library `odc-loader` to abstract the loading engine.

I could be worth merging the two concepts here, so that the higher-level functions in `odc-stac` work with STACGeoparquet indexes (shoudl be easy) and that the non-rio/gdal loading works as a backend enginer in `odc-stac` (less easy).

This is the Rasterio backend: https://github.com/opendatacube/odc-loader/blob/main/src/odc/loader/_rio.py

Start by reading the linked odc-loader/src/odc/loader/_rio.py backend and comparing it with odc-stac's higher-level functions. Determine the integration boundaries for STACGeoparquet indexes and non-Rasterio/GDAL loading; done requires an agreed, scoped merging plan.

Written by the indexing model from the issue text.