executor: display column collate in show create table

[tsthght]

What problem does this PR solve?

fix bug for #11831。should display collate of the column， but did not。

create table ttt4(a varchar(123) default null collate utf8mb4_unicode_ci)engine=innodb default charset=utf8mb4 collate=utf8mb4_unicode_ci;
show create table ttt4;
mysql> show create table ttt4\G
*************************** 1. row ***************************
Table: ttt4
Create Table: CREATE TABLE ttt4 (
a varchar(123) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
1 row in set (0.00 sec)

What is changed and how it works?

mysql> show create table ttt4\G
*************************** 1. row ***************************
Table: ttt4
Create Table: CREATE TABLE ttt4 (
a varchar(123) COLLATE utf8mb4_unicode_ci DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
1 row in set (0.00 sec)

Check List

Tests

• Unit test

[codecov]

Codecov Report

Merging #12327 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #12327   +/-   ##
===========================================
  Coverage   79.7595%   79.7595%           
===========================================
  Files           462        462           
  Lines        102216     102216           
===========================================
  Hits          81527      81527           
  Misses        14819      14819           
  Partials       5870       5870

[zz-jason]

@tsthght Please follow the Commit Message and Pull Request Style to refactor the PR title.

[zz-jason]

Also, please add a UT

[zz-jason]

LGTM

[zimulala]

/run-all-tests

[zimulala]

/rebuild

[zimulala]

LGTM

[zimulala]

/run-all-tests

[sre-bot]

/run-all-tests