Add surface field for highways

Keep surface field from OSM on highways, generalize it to two values:
"paved" and "unpaved".

This is a fix for #389 and a partial fix for #422.

layers/transportation/mapping.yaml

@@ -133,6 +133,10 @@ man_made_field: &man_made
 z_order_field: &z_order
   name: z_order
   type: wayzorder
+surface_field: &surface
+  key: surface
+  name: surface
+  type: string
 
 tables:
 # etldoc: imposm3 -> osm_highway_linestring
@@ -168,6 +172,7 @@ tables:
     - *usage
     - *public_transport
     - *man_made
+    - *surface
     mapping:
       highway:
       - motorway