point-cloud-import: Add `--override-crs` option

It seems common to find datasets which _really_ all have the same CRS but may have been processed by different pieces of software, and thus have small inconsistencies in their WKT Kart will reject imports of those datasets. However – especially when it has to reprocess the tiles anyway to convert to COPC – it'd be useful to be abel to ask it to override the CRS of tiles that don't match. Usage would be like: `kart point-cloud-import --override-crs=EPSG:1234 ...` Or presumably WKT could be supplied via `--override-crs=@myfile.wkt`