[client-go] Polymorphic Scale Client

This introduces a polymorphic scale client capable of operating against
scale subresources which return different group-versions of Scale.  The
scale subresources may be in group-versions different than the scale
itself, so that we no longer need a copy of every scalable resource in
the extensions API group.

To discovery which Scale group-versions go to which subresources,
discovery is used.

The scale client maintains its own internal versions and conversions to
several external versions, with a "hub" version that's a copy of the
autoscaling internal version.

It currently supports the following group-versions for Scale subresources:

- extensions/v1beta1.Scale
- autoscaling/v1.Scale

Kubernetes-commit: d61a2d90372c301dd11088df8941acf2bb01c38c

rest/url_utils.go

@@ -56,6 +56,14 @@ func DefaultServerURL(host, apiPath string, groupVersion schema.GroupVersion, de
 	// hostURL.Path should be blank.
 	//
 	// versionedAPIPath, a path relative to baseURL.Path, points to a versioned API base
+	versionedAPIPath := DefaultVersionedAPIPath(apiPath, groupVersion)
+
+	return hostURL, versionedAPIPath, nil
+}
+
+// DefaultVersionedAPIPathFor constructs the default path for the given group version, assuming the given
+// API path, following the standard conventions of the Kubernetes API.
+func DefaultVersionedAPIPath(apiPath string, groupVersion schema.GroupVersion) string {
 	versionedAPIPath := path.Join("/", apiPath)
 
 	// Add the version to the end of the path
@@ -64,10 +72,9 @@ func DefaultServerURL(host, apiPath string, groupVersion schema.GroupVersion, de
 
 	} else {
 		versionedAPIPath = path.Join(versionedAPIPath, groupVersion.Version)
-
 	}
 
-	return hostURL, versionedAPIPath, nil
+	return versionedAPIPath
 }
 
 // defaultServerUrlFor is shared between IsConfigTransportTLS and RESTClientFor. It