Support enable_google_cloud_serverless_authentication in Ingress annotation

[0anton]

Is your feature request related to a problem? Please describe.

Pomerium perfectly supports serverless backends in the standalone setup (docker). For some reason option enable_google_cloud_serverless_authentication is not allowed Ingress controller specification. This limits the ability of using Pomerium gateway operated in Kubernetes cluster to server serverless backends. The unique value proposition of the gateway to support serverless backends is therefore not realized through this limitation.

Describe the solution you'd like

I'd like to be able to enable passing over the serverless JWT token to configured destinations like I can do it in the standalone Pomerium.
ingress.pomerium.io/enable_google_cloud_serverless_authentication: 'true'

This can be realized by allowing the annotation ingress.pomerium.io/enable_google_cloud_serverless_authentication in the Ingress context:

apiVersion: v1
kind: Service
metadata:
  name: verify
  namespace: prod
spec:
  type: ExternalName
  externalName: verify-jigroeji-eq.run.app


apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: verify
  annotations:
    ingress.pomerium.io/pass_identity_headers: 'true'
    ingress.pomerium.io/enable_google_cloud_serverless_authentication: 'true'
spec:
  ingressClassName: pomerium
  rules:
    - host: 'verify.localhost.pomerium.io'
      http:
        paths:
          - pathType: Prefix
            path: /
            backend:
              service:
                name: verify
                port:
                  number: 443

Describe alternatives you've considered

Re-implementing Envoy custom path filter, which injects the serverless Bearer token in conjuction with other products, like Istio.

Explain any additional use-cases

Passing over Google JWT Bearer token enables wide set of hybrid backend scenaries / microservice architectures with the strong RBAC backend authentication/authorization. Not supporting authenticated backend calls forces leaving them unprotected, leading to worse security profiles.

Related to this.