Mark google_iam_oauth_client_credential client_secret as sensitive (GoogleCloudPlatform#13946)

Author: plus-1s

mmv1/products/iamworkforcepool/OauthClientCredential.yaml

@@ -85,6 +85,7 @@ properties:
       more, see [OAuth client and credential security risks and
       mitigations](https://cloud.google.com/iam/docs/workforce-oauth-app#security)
     output: true
+    sensitive: true
   - name: displayName
     type: String
     description: |-