Paper prototype on how HPA with custom KPA scaler may look like, without changing HPA api

hpa-kpa-v2.patch

--- a
+++ b
+---
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: php-apache
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: php-apache
  minReplicas: 1
  maxReplicas: 10
  metrics:
  - type: Resource
    resource:
      name: cpu
      target:
        type: Utilization
        averageUtilization: 50
+ - type: KPA
+   kpa:
+     name: requests_per_second
+     target:
+       type: Average
+       value: 100
+---
+apiVersion: autoscaling/v3
+kind: HorizontalPodAutoscalerConfiguration
+metadata:
+  name: kpa
+spec:
+  target:
+    metrics:
+     - KPA
+    allNamespaces: false
+    namespaces:
+     - foo
+     - bar
+  delegate:
+    scaler:
+      name: scaler-api
+      namespace: kpa
+    transport: bi-directional