Add aboriginal lands (#1489)

This PR adds support for `boundary=aboriginal_lands` by adding it to the `park` layer. While these lands are certainly not "parks", they have similar treatment from a technology perspective and can benefit from the existing processing chain established in that layer. I set all of these objects to `class=aboriginal_lands` in the tiles, including at the lowest zoom in order to separate it from the protected area merging implemented in #1160. In order to distinguish these from general parks, I expose the `class` attribute for these objects at z4 and also ensure that the z4 generalization is performed separately for protected areas versus aboriginal lands.

This unblocks #ZeLonewolf/openstreetmap-americana#105, which describes why having indigenous land boundaries is an important general feature on the map. This is also consistent with my suggested implementation in https://github.com/openmaptiles/openmaptiles/issues/1296#issuecomment-967749403.

layers/park/update_park_polygon.sql

@@ -24,14 +24,16 @@ DROP MATERIALIZED VIEW IF EXISTS osm_park_polygon_dissolve_z4 CASCADE;
 CREATE MATERIALIZED VIEW osm_park_polygon_dissolve_z4 AS
 (
   SELECT min(osm_id) AS osm_id,
-         ST_Union(geometry) AS geometry
+         ST_Union(geometry) AS geometry,
+         boundary
   FROM (
         SELECT ST_ClusterDBSCAN(geometry, 0, 1) OVER() AS cluster,
                osm_id,
-               geometry
+               geometry,
+               boundary
         FROM osm_park_polygon_gen_z4
   ) park_cluster
-  GROUP BY cluster
+  GROUP BY boundary, cluster
 );
 CREATE UNIQUE INDEX IF NOT EXISTS osm_park_polygon_dissolve_idx ON osm_park_polygon_dissolve_z4 (osm_id);