[Json] Support [*] in json path

[morningman]

This CL this just a test, not ready for reviewing

mysql> SELECT get_json_string('[{"k1":"v1"}, {"k2":"v2"}, {"k1":"v3"}, {"k1":"v4"}]', '$.[*].k1');
+-------------------------------------------------------------------------------------+
| get_json_string('[{"k1":"v1"}, {"k2":"v2"}, {"k1":"v3"}, {"k1":"v4"}]', '$.[*].k1') |
+-------------------------------------------------------------------------------------+
| ["v1","v3","v4"]                                                                    |
+-------------------------------------------------------------------------------------+
1 row in set (0.01 sec)