Add a preupload hook to forbid use of "required" keyword in protobuf

Test: make a commit which adds 'required' keyword to some fields in
update_metadata.proto, do repo upload, make sure preupload hook fails

Change-Id: I98296130dbea3c711482a7c9488a73d3005c15ba
diff --git a/PREUPLOAD.cfg b/PREUPLOAD.cfg
index 40ddcd1..85fd5ec 100644
--- a/PREUPLOAD.cfg
+++ b/PREUPLOAD.cfg
@@ -2,3 +2,6 @@
 clang_format = true
 cpplint = true
 pylint = true
+
+[Hook Scripts]
+protobuflint = ./protobuflint.py ${PREUPLOAD_COMMIT} ${PREUPLOAD_FILES}