Smarter geometry loading from `DataCube.aggregate_spatial`

[soxofaan]

DataCube.aggregate_spatial(geometries) currently allows geometries to be a string or pathlib.Path, but then it assumes this is a backend-side path and "loads" this with non-standard read_vector.

It would be better (more user friendly, and more standard) to be a bit smarter here:

• check if the string is a GeoJSON dump and load it like that
• check if the string/path is a valid local (GeoJSON) file and load geometry from that

Also

• start deprecating the current read_vector code path
• share this feature with other geometry related processes
• document this feature better

[soxofaan]

(this was triggered from https://discuss.eodc.eu/t/aggregate-spatial/766)