We were repeating that logic a lot, so this helper just reads a policy,
calls the passed modify function on the policy, then writes the policy
back and takes care of the optimistic concurrency logic for the caller.
So now all the caller has to do is the unique part, which is the modify
function.
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.
