Merge pull request #36033 from DirectXMan12/feature/hpa-v2

Automatic merge from submit-queue (batch tested with PRs 40796, 40878, 36033, 40838, 41210)

HPA v2 (API Changes)

**Release note**:
```release-note
Introduces an new alpha version of the Horizontal Pod Autoscaler including expanded support for specifying metrics.
```

Implements the API changes for kubernetes/features#117.

This implements #34754, which is the new design for the Horizontal Pod Autoscaler.  It includes improved support for custom metrics (and/or arbitrary metrics) as well as expanded support for resource metrics.  The new HPA object is introduces in the API group "autoscaling/v1alpha1".

Note that the improved custom metric support currently is limited to per pod metrics from Heapster -- attempting to use the new "object metrics" will simply result in an error.  This will change once #34586 is merged and implemented.

federation/apis/openapi-spec/swagger.json

@@ -4816,6 +4816,39 @@
      }
     ]
    },
+   "/apis/autoscaling/v2alpha1/": {
+    "get": {
+     "description": "get available resources",
+     "consumes": [
+      "application/json",
+      "application/yaml",
+      "application/vnd.kubernetes.protobuf"
+     ],
+     "produces": [
+      "application/json",
+      "application/yaml",
+      "application/vnd.kubernetes.protobuf"
+     ],
+     "schemes": [
+      "https"
+     ],
+     "tags": [
+      "autoscaling_v2alpha1"
+     ],
+     "operationId": "getAutoscalingV2alpha1APIResources",
+     "responses": {
+      "200": {
+       "description": "OK",
+       "schema": {
+        "$ref": "#/definitions/io.k8s.apimachinery.pkg.apis.meta.v1.APIResourceList"
+       }
+      },
+      "401": {
+       "description": "Unauthorized"
+      }
+     }
+    }
+   },
    "/apis/batch/": {
     "get": {
      "description": "get information of a group",