Add COG validation endpoint

[banesullivan]

Large-image's GDAL source has a validateCOG method which should be wrapped into an endpoint to validate if an image is a COG

https://girder.github.io/large_image/_build/large_image_source_gdal/large_image_source_gdal.html#large_image_source_gdal.GDALFileTileSource.validateCOG

This will not be available/valid for all tile sources, so we will need a clever way to limit it to GDAL sources and error out when not a GDAL source