The "extra" field in the result of "show columns" with default value expressions is not processed

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Related issue：#50936

create table t2 (c int, c1 double default (rand(1)));
show columns from test.t2 where field='c1';

2. What did you expect to see? (Required)

mysql> show columns from test.t2 where field='c1';
+-------+--------+------+-----+---------+-------------------+
| Field | Type   | Null | Key | Default | Extra             |
+-------+--------+------+-----+---------+-------------------+
| c1    | double | YES  |     | rand(1) | DEFAULT_GENERATED |
+-------+--------+------+-----+---------+-------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

tidb> show columns from test.t2 where field='c1';
+-------+--------+------+------+---------+-------+
| Field | Type   | Null | Key  | Default | Extra |
+-------+--------+------+------+---------+-------+
| c1    | double | YES  |      | rand(1) |       |
+-------+--------+------+------+---------+-------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

master