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>

manifest/schema2/manifest.go

@@ -20,6 +20,10 @@ const (
 	// MediaTypeLayer is the mediaType used for layers referenced by the
 	// manifest.
 	MediaTypeLayer = "application/vnd.docker.image.rootfs.diff.tar.gzip"
+
+	// MediaTypeForeignLayer is the mediaType used for layers that must be
+	// downloaded from foreign URLs.
+	MediaTypeForeignLayer = "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip"
 )
 
 var (
@@ -63,7 +67,6 @@ type Manifest struct {
 // References returnes the descriptors of this manifests references.
 func (m Manifest) References() []distribution.Descriptor {
 	return m.Layers
-
 }
 
 // Target returns the target of this signed manifest.