Unified GeometryArray Arrow union type without generic

[kylebarron]

Right now all native-encoded arrays have a parameterization. And they are parameterized over both geometry type and dimension.

Arc<dyn NativeArray> doesn't have a parameterization, but it isn't an arrow type, it's a Rust concept on top of Arrow-native arrays. You couldn't define an Arc<dyn NativeArray> in an Arrow schema.

But we could have a union type that contains all geometry types and all dimensions. Ref geoarrow/geoarrow#43 (comment)

Then we can always represent any arbitrary Geometry input, while still having a fixed schema.