Add a resource that manages just a single binding within a Google
project's IAM Policy.
Note that this resource should not be used when
google_project_iam_policy is used, or they will fight over which is
correct.
This also required wrapping the error returned from setProjectIamPolicy,
as we need to test to see if it's a 409 error and retry, which can't be
done if we just use fmt.Errorf.
