gke-gcloud-auth-plugin: Support passing credentials via a specific JSON filepath

[Lawouach]

Hi all,

Today, when you use google-cloud-sdk-gke-gcloud-auth-plugin you can rely on GOOGLE_APPLICATION_CREDENTIALS via the --use_application_default_credentials flag. However, the GOOGLE_APPLICATION_CREDENTIALS is explicitely forbidden to be set in a Cloud Run by the documentation https://cloud.google.com/run/docs/securing/service-identity.

This means if you try to create a Cloud Run job that needs to connect to a GKE cluster, you have to either dismiss the documentation warning or find a way to manually create the kubeconfig with all the information.

Would it be possible to add a flag such as ``--application_default_credentials_file` which, when not set, would default to the usual places, but when set would pick up the file.

[k8s-ci-robot]

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[BenTheElder]

/retitle gke-gcloud-auth-plugin: Support passing credentials via a specific JSON filepath