feat: add quota project to base credentials class (#546)

diff --git a/tests/test_iam.py b/tests/test_iam.py
index ea7d08a..e20eeba 100644
--- a/tests/test_iam.py
+++ b/tests/test_iam.py
@@ -50,6 +50,9 @@
         def refresh(self, request):
             pass
 
+        def with_quota_project(self, quota_project_id):
+            raise NotImplementedError()
+
     return CredentialsImpl()