KMSCNG: Please make the path to config.yaml configurable

I have an application which tries to deal with the public key versions. I can load the correct version of a key to use dynamically. However, this application is not running as Administrator, so it cannot automatically create or edit files in `C:\Windows\KMSCNG`. Could you add an environment variable to allow overriding the hardcoded path to `C:\Windows\KMSCNG\config.yaml` described at https://github.com/GoogleCloudPlatform/kms-integrations/blob/master/kmscng/docs/user_guide.md#configuration ?

Thanks.