Add option for configuring HPA with custom/external metrics

[locmai]

What this PR does:

Add an option to auto-scale on custom and external metrics.

Which issue(s) this PR fixes:
No issue opened yet

Test values.yaml:

nginx:
  autoscaling:
    enabled: true
    customMetrics:
    - type: External
      external:
        metric:
          name: queue_messages_ready
          selector:
            matchLabels:
              queue: "worker_tasks"
        target:
          type: AverageValue
          averageValue: 30
    - type: External
      external:
        metric:
          name: queue_workers_ready
        target:
          type: AverageValue
          averageValue: 10

Result:

---
# Source: cortex/templates/nginx/nginx-hpa.yaml
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: release-name-cortex-nginx
  namespace: default
  labels:
    helm.sh/chart: cortex-2.1.0
    app.kubernetes.io/name: cortex
    app.kubernetes.io/instance: release-name
    app.kubernetes.io/version: "v1.15.1"
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/component: nginx
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: release-name-cortex-nginx
  minReplicas: 2
  maxReplicas: 30
  metrics:
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 80
    - external:
        metric:
          name: queue_messages_ready
          selector:
            matchLabels:
              queue: worker_tasks
        target:
          averageValue: 30
          type: AverageValue
      type: External
    - external:
        metric:
          name: queue_workers_ready
        target:
          averageValue: 10
          type: AverageValue
      type: External
---

Checklist

• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[locmai]

Hi @nschad and @kd7lxl , could you take a look at this PR?

[nschad]

thanks!