Add `user_agent` property to `ClientInfo` (#7799)

* Add `user_agent` property to `ClientInfo`

This provides a way for partners to define a prefix identifying their
tool or application, as required by many cloud partnership agreements.

* Workaround for pytype pyi error with nested class.
diff --git a/setup.cfg b/setup.cfg
index 581d862..5c32e16 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -8,3 +8,5 @@
 exclude =
     tests/
 output = pytype_output/
+# Workaround for https://github.com/google/pytype/issues/150
+disable = pyi-error