Add support for layers from foreign sources

This will be used to support downloading Windows base layers from
Microsoft URLs.

Signed-off-by: John Starks <jostarks@microsoft.com>

docs/spec/manifest-v2-2.md

@@ -216,6 +216,14 @@ image. It's the direct replacement for the schema-1 manifest.
         The digest of the content, as defined by the
         [Registry V2 HTTP API Specificiation](https://docs.docker.com/registry/spec/api/#digest-parameter).
 
+    - **`urls`** *array*
+
+        For an ordinary layer, this is empty, and the layer contents can be
+        retrieved directly from the registry. For a layer with *`mediatype`* of
+        `application/vnd.docker.image.rootfs.foreign.diff.tar.gzip`, this
+        contains a non-empty list of URLs from which this object can be
+        downloaded.
+
 ## Example Image Manifest
 
 *Example showing an image manifest:*