feature(tianmu): to support explain the table of Tianmu engine

[DandreChen]

Is your feature request related to a problem? Please describe.

case：

create table ttt(a varchar(20)) engine=tianmu;
insert into ttt values("123"),("456"),("789");
explain  select * from ttt;

result:

mysql> explain  select * from ttt;
+------+
| a    |
+------+
| 123  |
| 456  |
| 789  |
+------+
3 rows in set (2.80 sec)

Describe the solution you'd like

fix tianmu code to judge the explain operation

Describe alternatives you've considered

Additional context