feat: add a few less common unpaved surface types

Ways labelled with OSM tag `surface` set to "rock", "rocks", "stone", "shells" or "salt" are classified as surface type "Unpaved" rather than "Unknown".

ors-engine/src/main/java/org/heigit/ors/routing/graphhopper/extensions/SurfaceType.java

@@ -59,7 +59,7 @@ public static SurfaceType getFromString(String surface) {
 
  return switch (surface.toLowerCase()) {
  case "paved" -> SurfaceType.PAVED;
- case "unpaved", "woodchips" -> SurfaceType.UNPAVED;
+ case "unpaved", "woodchips", "rock", "rocks", "stone", "shells", "salt" -> SurfaceType.UNPAVED;
  case "asphalt" -> SurfaceType.ASPHALT;
  case "concrete" -> SurfaceType.CONCRETE;
  case "paving_stones", "paved_stones", "sett", "cobblestone", "unhewn_cobblestone", "bricks", "brick" -> SurfaceType.PAVING_STONE;