[Alter] Support alter table and column's comment

[morningman]

Proposed changes

1. alter table tbl1 modify comment "new comment";
2. alter table tbl1 modify column k1 comment "k1", modify column v1 comment "v1";

Types of changes

What types of changes does your code introduce to Doris?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)
• Code refactor (Modify the code structure, format the code, etc...)
• Optimization. Including functional usability improvements and performance improvements.
• Dependency. Such as changes related to third-party components.
• Other.

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have created an issue on (Fix [Feature] Support alter column and table's comment #6409 ) and described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• If these changes need document changes, I have updated the document
• Any dependent changes have been merged

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[xy720]

LGTM

[github-actions]

PR approved by anyone and no changes requested.

[EmmyMiao87]

Please add a issue link

[EmmyMiao87]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.