Added build area assessments vector tiles function

\approve @nicgar

migrations/00011_add-built-area-assessments-function.sql

+---- Generates vector tiles with values from the `obm_tiles` database table.
+
+CREATE OR REPLACE FUNCTION public.obm_built_area_assessments(z INTEGER, x INTEGER, y INTEGER)
+RETURNS BYTEA AS
+$$
+    DECLARE
+        vector_tile BYTEA DEFAULT '';
+        parent_quadkey TEXT;
+        inside_tiles INTEGER; -- Number of tiles at zoom level 18 on an axis of a tile with zoom level `z`.
+        inside_tile_pixels INTEGER;
+    BEGIN
+        IF z > 18 THEN
+            RAISE NOTICE 'Vector tiles cannot be be created for zoom levels higher than 18.';
+        ELSE
+            parent_quadkey = convert_xyz_to_quadkey(x, y, z);
+            inside_tiles = 2 ^ (18 - z);
+            inside_tile_pixels = 4096 / inside_tiles;
+            WITH mvtdata AS (
+                SELECT ST_MakeEnvelope(
+                    inside_tile_pixels * tile_x, inside_tile_pixels * tile_y,
+                    inside_tile_pixels * (tile_x + 1), inside_tile_pixels * (tile_y + 1)
+                ) AS geom,
+                tiles.quadkey,
+                tiles.source_id,
+                tiles.built_area_size,
+                tiles.built_up_ratio,
+                tiles.last_update
+                FROM
+                    obm_built_area_assessments tiles,
+                    generate_series(0, inside_tiles - 1) AS tile_x,
+                    generate_series(0, inside_tiles - 1) AS tile_y
+                WHERE tiles.quadkey
+                    = parent_quadkey || convert_xyz_to_quadkey(tile_x, tile_y, 18 - z)
+                AND tiles.source_id = 1
+            )
+            SELECT ST_AsMVT(mvtdata.*, 'obm_tiles', 4096, 'geom')
+            FROM mvtdata INTO vector_tile;
+        END IF;
+        RETURN vector_tile;
+    END;
+$$
+LANGUAGE plpgsql STABLE PARALLEL SAFE;