[Bug]information_schema.columns table column_comment does not show

[Astralidea]

Describe the bug
A clear and concise description of what the bug is.

场景：从应用的角度来讲，用户可以不用通过show create table的解析拿到comment.
一些框架也会通过这个拿comment。
虽然api也能拿到comment，但是有些时候可能不会开放http api。

To Reproduce
Steps to reproduce the behavior:

1. create any table with column comment.
2. select * from information_schema.columns;
3. column_comment does not show comment

Expected behavior
A clear and concise description of what you expected to happen.
show comment

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.
已经定位到了原因，后面会提个PR。