plugins exts CHANGE ext parsing isolated into a callback

Lots of refactoring and finishing up included.
diff --git a/src/parser_json.c b/src/parser_json.c
index 3badd6c..2565c12 100644
--- a/src/parser_json.c
+++ b/src/parser_json.c
@@ -926,7 +926,7 @@
     if (*node_p) {
         /* insert, keep first pointer correct */
         if (ext) {
-            lyd_insert_ext(parent, *node_p);
+            lyplg_ext_insert(parent, *node_p);
         } else {
             lyd_insert_node(parent, first_p, *node_p, last);
         }