chore(prefect-worker): add CPU/memory-based HPA

[parkedwards]

resolves #335

---
# Source: prefect-worker/templates/hpa.yaml
apiVersion: autoscaling/v2
kind: HorizontalPodAutoscaler
metadata:
  name: prefect-worker
  labels:
    app.kubernetes.io/name: prefect-worker
    helm.sh/chart: prefect-worker-0.0.0
    app.kubernetes.io/instance: release-name
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/component: worker
    prefect-version: 2-latest
spec:
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: prefect-worker
  minReplicas: 1
  maxReplicas: 1
  metrics:
    - type: Resource
      resource:
        name: memory
        target:
          type: Utilization
          averageUtilization: 80
    - type: Resource
      resource:
        name: cpu
        target:
          type: Utilization
          averageUtilization: 80

[parkedwards]

this form key doesn't actually adhere to the JSON schema, so im suspecting it doesnt do anything here. im going to verify and clean this out in a followup PR

[jamiezieziula]

ty