[FEATURE] Add basic geometry attributes for custom schema inline expressions

[quincylvania]

Is your feature request related to a problem? Please describe.
For OpenTrailMap I'd like to be able to be able to show the trail length. I'd also like to filter parks by size/coverage with MapLibre instead of with min_size in the tiles (gets around our need for #965 too).

Describe the solution you'd like
I'd like to be able to do something like the below. I'm not sure about the best syntax or property names, or if these should be functions instead.

attributes: 
  - key: length
    value: '${ feature.length_km }'
  - key: area
    value: '${ feature.area_km2 }'
  - key: bbox_area
    value: '${ feature.bbox_area_km2 }'
  - key: min_lat
    value: '${ feature.min_lat }'
  - key: min_lon
    value: '${ feature.min_lon }'
  - key: max_lat
    value: '${ feature.max_lat }'
  - key: max_lon
    value: '${ feature.max_lon }'

For point features the area/length would return zero and the min/max lat/lon would just be the feature's lat/lon.

[msbarry]

OK got it, these should be relatively straightforward to add although I might want to find a way to handle other units (km, miles, feet, acres, hectares, etc.) Also I don't think anything has had to compute exact distances between lat/lon yet so will need to add in that support to the java layer as well.

[quincylvania]

I might want to find a way to handle other units (km, miles, feet, acres, hectares, etc.)

This might be helpful. Though in MapLibre you can apply multiplication expressions to attributes before displaying them as labels, so developer can easily apply conversions themselves.

In my proposal I have the default units at km and km2, but they should probably be m and m2 instead to better accommodate more human-sized features.

[msbarry]

Take a look at #1084 and let me know what you think - it exposes all of the geometry utilities that java profiles have access to to inline expressions so I think it should cover these cases.

[quincylvania]

@msbarry Looks perfect for our use case, thanks!