data types CHANGE printer plugin

Change API to be able to support printing data values even in non-data
formats (YIN/YANG) using callback to provide prefixes to use in the
value. This will allow to change format of the default values in schemas
to unify it with the data tree.
diff --git a/src/common.h b/src/common.h
index 718a83b..68b0770 100644
--- a/src/common.h
+++ b/src/common.h
@@ -420,6 +420,14 @@
                                    const char **value, size_t *value_len, const char **errmsg);
 
 /**
+ * @brief ly_clb_get_prefix implementation for JSON. For its simplicity, this implementation is used
+ * internally for various purposes.
+ *
+ * Implemented in printer_json.c
+ */
+const char *json_print_get_prefix(const struct lys_module *mod, void *private);
+
+/**
  * @brief mmap(2) wrapper to map input files into memory to unify parsing.
  *
  * The address space is allocate only for reading.