Add metrics to horizontalPodAutoscalerConfig to support custom metrics api

[imroc]

Add .spec.advanced.horizontalPodAutoscalerConfig.metrics to ScaledObject, so that we can keep some metric specs that uses custom metrics api to auto scale workloads.

For example, a HPA uses custom metrics that provided by cloud vendor's hosted kubernetes cluster (e.g. Tencent TKE's HPA metrics)

apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: test
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: test
  minReplicas: 1
  maxReplicas: 100
  metrics:
    - pods:
        metric:
          name: k8s_pod_rate_cpu_core_used_limit
        target:
          averageValue: "80"
          type: AverageValue
      type: Pods
    - pods:
        metric:
          name: k8s_pod_rate_mem_usage_limit
        target:
          averageValue: "80"
          type: AverageValue
      type: Pods
    - pods:
        metric:
          name: k8s_pod_rate_gpu_used_request
        target:
          averageValue: "60"
          type: AverageValue
      type: Pods

With this PR, we can migrate this HPA to KEDA, and add the KEDA trigger to enhance the autoscaling ability:

apiVersion: keda.sh/v1alpha1
kind: ScaledObject
metadata:
  name: test
  namespace: test
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: test
  pollingInterval: 15
  minReplicaCount: 1
  maxReplicaCount: 100
  advanced:
    horizontalPodAutoscalerConfig:
      metrics:
        - pods:
            metric:
              name: k8s_pod_rate_cpu_core_used_limit
            target:
              averageValue: "80"
              type: AverageValue
          type: Pods
        - pods:
            metric:
              name: k8s_pod_rate_mem_usage_limit
            target:
              averageValue: "80"
              type: AverageValue
          type: Pods
        - pods:
            metric:
              name: k8s_pod_rate_gpu_used_request
            target:
              averageValue: "60"
              type: AverageValue
          type: Pods
  triggers:
    - type: cron
      metadata:
        timezone: Asia/Shanghai
        start: 30 9 * * *
        end: 30 10 * * *
        desiredReplicas: "10"

See background: #5810