x509 cert fix

[modular-magician]

b/453727810

attribute from cert json used to determine x509 eligibility based on https://docs.cloud.google.com/go/docs/reference/cloud.google.com/go/auth/latest/credentials/externalaccount#cloud_google_com_go_auth_credentials_externalaccount_CredentialSource

Tested using a basic network

resource "google_compute_network" "custom_test" {
  provider = google-beta
  project = "terraform-dev-nickelliot-xxx"
  name                    = "tf-test-x509-nickelliot"
  auto_create_subnetworks = false
}

This run using my config for an X.509 Cert, using a config setup following the steps from the official docs here: https://docs.cloud.google.com/iam/docs/workload-identity-federation-with-x509-certificates
gpaste/4807609722994688

This run using my standard set-up (does not contain the debug lines added for X.509's auth flow)
gpaste/6295676673720320

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provider: an issue preventing X.509 certificates from being used for authentication when supplied as Application Default Credentials as been resolved

Derived from GoogleCloudPlatform/magic-modules#15691