Feature request: Allow using GOOGLE_CREDENTIALS environment variable

[ericnorris]

Thanks for releasing this! We're somewhat interested in exploring using this provider while we wait for some updates in the Cloud Identity APIs, but we noticed that it currently uses a different set of environment variables for credentials from the google provider.

Is there any chance this provider could support the use of the GOOGLE_CREDENTIALS environment variable? Our Terraform Cloud workspaces are already configured with this environment variable, and doing so would allow us to use this provider with no further changes.