FlatGeobuf 3d support

[kylebarron]

This is failing on unimplemented 3d downcasting

the z dimension of points is getting lost

            columns: [
                PrimitiveArray<UInt8>
                [
                  3,
                  2,
                  1,
                ],
                StringArray
                [
                  "baz",
                  "bar",
                  "foo",
                ],
                FixedSizeListArray<3>
                [
                  PrimitiveArray<Float64>
                [
                  6.0,
                  7.0,
                  NaN,
                ],
                  PrimitiveArray<Float64>
                [
                  3.0,
                  4.0,
                  NaN,
                ],
                  PrimitiveArray<Float64>
                [
                  0.0,
                  1.0,
                  NaN,
                ],
                ],
            ],