Add binary authorization setting to GKE cluster

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This MR adds a new variable gke_binary_authorization_evaluation_mode that controls the binary authorization evaluation mode for the GKE cluster. By default this variable has a value of DISABLED, meaning binary authorization is not enabled on the cluster. This variable can set to PROJECT_SINGLETON_POLICY_ENFORCE to enabled Binary Authorization on the cluster. This causes the cluster to conform to the GCP project's binary authorization policy.