Discover BIFROST_GOOGLE_IAP_BACKEND_SERVICE_ID automatically

[Starefossen]

Currently this variable is set manually from Fasit as we don't have an easy way to find it.

https://github.com/nais/fasit/blob/main/features/bifrost.yaml#L15-L18

config:
  backend.google.iapBackendServiceId:
    type: string
    description: ID for Backend Service for IAP loadbanacer used for verifying JWT tokens from IAP.

Google describes how to do this manually using gcloud, since this is created via an ingress it is a little more "unknown" what the name will be.

gcloud compute backend-services describe SERVICE_NAME --project=PROJECT_ID --global