Julia geometry ecosystem integration

The geometry ecosystem is maturing, and we should take a fresh look at whether and how to integrate with it. Development seems to focus on geosciences but there are still generic methods and abstractions that could be useful to us. Probably the most utility for the least work would be to [implement GeoInterface](https://juliageo.org/GeoInterface.jl/stable/guides/developer/) for our `Polygon`. Then we could at least use things like [GeometryOps](https://github.com/JuliaGeo/GeometryOps.jl) (e.g. for pure Julia clipping and [simplification](https://juliageo.org/GeometryOps.jl/stable/source/transformations/simplify)) or [Rasters.jl](https://github.com/rafaqz/Rasters.jl/) (c.f. our homebrew `gridpoints_in_polygon`) with polygon-only geometries.

Start by reading the Polygon implementation and the existing homebrew gridpoints_in_polygon functionality, then review the linked GeoInterface developer guide. Determine the interface needed for Polygon and check the proposed GeometryOps clipping/simplification and Rasters.jl polygon use cases; done means Polygon integrates with the relevant generic methods.

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