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

tests/test_credentials.py

diff --git a/tests/test_credentials.py b/tests/test_credentials.py
index 16ddd9b..2023fac 100644
--- a/tests/test_credentials.py
+++ b/tests/test_credentials.py
@@ -24,6 +24,9 @@
     def refresh(self, request):
         self.token = request
 
+    def with_quota_project(self, quota_project_id):
+        raise NotImplementedError()
+
 
 def test_credentials_constructor():
     credentials = CredentialsImpl()
@@ -112,6 +115,12 @@
     assert headers == {}
 
 
+def test_anonymous_credentials_with_quota_project():
+    with pytest.raises(ValueError):
+        anon = credentials.AnonymousCredentials()
+        anon.with_quota_project("project-foo")
+
+
 class ReadOnlyScopedCredentialsImpl(credentials.ReadOnlyScoped, CredentialsImpl):
     @property
     def requires_scopes(self):