Expand Jiyong's apex OWNERship

There's more to apex than just apex.go

Change-Id: I6603db10c43148c4f17fb9d3b762400472b32526
diff --git a/apex/OWNERS b/apex/OWNERS
new file mode 100644
index 0000000..a382ae8
--- /dev/null
+++ b/apex/OWNERS
@@ -0,0 +1 @@
+per-file * = jiyong@google.com
\ No newline at end of file