plugins_exts FEATURE added printer_tree callbacks

A new interface has been added that allows the plugin to add nodes
to print a YANG tree diagram and also change the appearance of nodes.
diff --git a/src/plugins_exts/metadata.c b/src/plugins_exts/metadata.c
index 32fbe7d..9567e07 100644
--- a/src/plugins_exts/metadata.c
+++ b/src/plugins_exts/metadata.c
@@ -231,6 +231,8 @@
         .plugin.parse = annotation_parse,
         .plugin.compile = annotation_compile,
         .plugin.printer_info = annotation_printer_info,
+        .plugin.printer_ctree = NULL,
+        .plugin.printer_ptree = NULL,
         .plugin.node = NULL,
         .plugin.snode = NULL,
         .plugin.validate = NULL,