Backport to branch(3) : Support ifNotExists in add column

[feeblefakie]

This is an automated request for a manual backport of the following:

• Original PR: Support ifNotExists in add column #2960
• Commit to backport: 1064cf8

1. Resolve any conflicts that occur during the cherry-picking process.

git fetch origin &&
git checkout 3-pull-2960 &&
git cherry-pick --no-rerere-autoupdate -m1 1064cf8323023bbba96d1e9b123b7071d7d98e06

2. Push the changes.
3. Merge this PR after all checks have passed.

Thank you!