[ISSUE-2493] Add SQL syntax for metadata queries(show child nodes path)

[Yuxiang Song]

Hi all :


   I have submitted a pr for 
[ISSUE-2493](https://github.com/apache/iotdb/issues/2493), please take a look.
   The modules involved are as follows:
      1??antlr
      2??server/query planner 
      3??server/metadata


   Best,
   Yuxiang