Auth: Service account token override by JWT auth

### What happened? Our Grafana instance is behind an identity proxy ([Google IAP](https://cloud.google.com/security/products/iap)), which forward JWT headers to our Grafana instances. It's work perfectly well for our users, but we also have Grafana service account with a token for Terraform. There is no issue to pass the identity proxy via Terraform and set the Grafana service account token in the authorization header but unfortunately JWT headers are also pass to the Grafana instance along with the Authorization header. It's seems JWT headers takes precedence on authorization header. So Grafana doesn't use the Grafana service account for the calls to the API but the Google service account use to pass the identity proxy ```hcl provider "grafana" { url = var.grafana_url auth = data.google_secret_manager_secret_version_access.grafana_token.secret_data http_headers = { "Proxy-Authorization" : format("Bearer %s", ephemeral.google_service_account_id_token.terraform.id_token) } } ``` Using this, Terraform is able to communicate with the Grafana instance, but authentification in Grafana is the google service account used by terraform, and not the Grafana service account. In fact, I can even set `auth = "invalid-token"` and still be able to communicate with Grafana API. In my Grafana users list, I can see a user with the email from my Google service account (the one used by Terraform) ### What did you expect to happen? Ability to use our Grafana service account, instead of the Google service account used to pass the identity proxy. ### Did this work before? Never try on previous version ### How do we reproduce it? 1. Configure a Grafana instance with jwt auth 2. Configure a Grafana service account 3. Configure Google IAP in front of Grafana 4. Configure a Google service account with permission to call IAP backend 5. Call the Grafana API using Google service account to auth to the identity proxy, and the Grafana service account to auth to the Grafana API ### Is …