Reduce code duplication for IAP config

[JoepdeJong]

Currently, the following config is needed to setup a page behind IAP:

---
apiVersion: networking.gke.io/v1
kind: GCPBackendPolicy
metadata:
  name: dienst2-backend-policy
  namespace: default
spec:
  default:
    iap:
      enabled: true
      oauth2ClientSecret:
        name: iap-secret
      clientID: 966138216790-us6sue108a879v3vk2om9inn3ftpobkg.apps.googleusercontent.com
  targetRef:
    group: ""
    kind: Service
    name: dienst2
---
apiVersion: gateway.networking.k8s.io/v1beta1
kind: HTTPRoute
metadata:
  name: dienst2-route
  namespace: default
spec:
  parentRefs:
  - kind: Gateway
    name: gateway
  hostnames:
  - dienst2.wisv.ch
  rules:
  - backendRefs:
    - name: dienst2
      port: 80
---
apiVersion: v1
kind: Service
metadata:
  name: dienst2
  namespace: default
spec:
  ports:
  - name: http
    protocol: TCP
    port: 80
    targetPort: 8000
  selector:
    run: dienst2
  type: ClusterIP
---
apiVersion: networking.gke.io/v1
kind: HealthCheckPolicy
metadata:
  namespace: default
  name: dienst2
spec:
  default:
    config:
      type: HTTP
      httpHealthCheck:
        port: 8000
        requestPath: /healthz
    logConfig:
      enabled: true
  targetRef:
    group: ""
    kind: Service
    name: dienst2

Maybe we can make a default template for this?

[JoepdeJong]

IAP Chart? @julian9499