fix: Support parsing ESRI WKT String for ESRI-created custom CRS

[kylebarron]

Ref source-cooperative/cog-viewer#8 and developmentseed/geotiff-test-data#58 and developmentseed/async-geotiff#165. This GeoTIFF includes

GeoTIFF has model_type, geographic_type, projected_type geokeys.

• If model_type is 1, check the projected_type
  • If projected_type is an int < 32767, it's an EPSG code; else CRS encoded in GeoKeys
• If model_type is 2, check the geographic_type
  • If geographic_type is an int < 32767, it's an EPSG code; else CRS encoded in GeoKeys
• If model_type is 32767, then the projection is "undefined", and there's a (hack? grandfathered?) way that uses ESRI WKT in the proj citation key.

This PR now supports the latter approach when model_type itself is 32767.

[kylebarron]

The image still doesn't render but this is a valid improvement (with a test), so we can move the rendering question to the next branch.