[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

scale/interfaces.go

@@ -0,0 +1,39 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package scale
+
+import (
+	autoscalingapi "k8s.io/api/autoscaling/v1"
+	"k8s.io/apimachinery/pkg/runtime/schema"
+)
+
+// ScalesGetter can produce a ScaleInterface
+// for a particular namespace.
+type ScalesGetter interface {
+	Scales(namespace string) ScaleInterface
+}
+
+// ScaleInterface can fetch and update scales for
+// resources in a particular namespace which implement
+// the scale subresource.
+type ScaleInterface interface {
+	// Get fetches the scale of the given scalable resource.
+	Get(resource schema.GroupResource, name string) (*autoscalingapi.Scale, error)
+
+	// Update updates the scale of the the given scalable resource.
+	Update(resource schema.GroupResource, scale *autoscalingapi.Scale) (*autoscalingapi.Scale, error)
+}