self-hosted/docs: GKE reference architecture doesn't create external-dns secret

[adrienthebo]

Is your feature request related to a problem? Please describe

The GKE reference architecture describes how to setup external-dns with a newly created managed zone and provides a secret key under which GCP service accounts can be found, but doesn't create a Kubernetes secret that the external-dns service account can access (as the only secret created is in the cert-manager namespace). This creates an external-dns installation without the necessary credentials for interacting with the Cloud DNS API.

Describe the behaviour you'd like

The GKE reference architecture should explicitly create a Kubernetes secret with the GCP Cloud DNS credentials and reference that secret when installing the external-dns helm chart.

[stale]

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