feat: add quota_project, credentials_file, and scopes support (#1022)

Add support for client options:
* quota_project_id
* credentials_file
* scopes

These are only available when default credentials are used.
diff --git a/noxfile.py b/noxfile.py
index d5bc2b3..1f95edf 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -44,7 +44,7 @@
     )
 
 
-@nox.session(python=["2.7", "3.5", "3.6", "3.7"])
+@nox.session(python=["2.7", "3.5", "3.6", "3.7", "3.8"])
 @nox.parametrize(
     "oauth2client",
     [