feat: Add custom scopes for access tokens from the metadata service (#633)

This works for App Engine, Cloud Run and Flex. On Compute Engine you
can request custom scopes, but they are ignored.

Co-authored-by: Tres Seaver <tseaver@palladion.com>
Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>
diff --git a/system_tests/noxfile.py b/system_tests/noxfile.py
index 0f852ea..699a1b3 100644
--- a/system_tests/noxfile.py
+++ b/system_tests/noxfile.py
@@ -315,7 +315,7 @@
     session.env[EXPECT_PROJECT_ENV] = "1"
     session.install(*(TEST_DEPENDENCIES_SYNC + TEST_DEPENDENCIES_ASYNC))
     session.install(LIBRARY_DIR)
-    session.run("pytest", "system_tests_async/test_default.py", 
+    session.run("pytest", "system_tests_async/test_default.py",
     "system_tests_async/test_id_token.py")