Implement private road tagging (#1174)

Fixes #1066 

This PR adds a new field `access` in the transportation layer, which will be set to `no` if the `access` tag is either `no` or `private`.  While `private` is the more popular value by a 17:1 ratio, I went with `no` because it's smaller in the tiles.  In addition, the text `no` opens up the future possibility of other text-based access values such as `destination`, `customers`, or `permit`.

The screenshot below shows an example of access tagging for a road on a military base:

![image](https://user-images.githubusercontent.com/3254090/129431491-9acbaeca-bf18-4384-8177-2c198834865c.png)

layers/transportation/mapping.yaml

@@ -92,6 +92,10 @@ service_field: &service
   key: service
   name: service
   type: string
+access_field: &access
+  key: access
+  name: access
+  type: string
 usage_field: &usage
   key: usage
   name: usage
@@ -183,6 +187,7 @@ tables:
     - *oneway
     - *area
     - *service
+    - *access
     - *usage
     - *public_transport
     - *man_made