Slack: [Feature Request] Create notification on label change

[rnhurt]

In our Slack integration we limit notifications to a certain label. Creating the PR and then changing the label doesn't trigger a notification. It doesn't even trigger the notification if you add a comment later.

Here are the steps to recreate the problem:
1) subscribe to a repo with +label:ready-for-review
2) create a PR without a label
3) attach the label "ready-for-review"
4) notice that there is no notification in Slack for this PR
5) update the PR comment
6) notice that there is still no notification in Slack for this PR

NOTE: This is similar to these issues: #963, #961, #832

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[alvin-huang]

This is especially important for OSS projects where contributors have no permissions to add labels and labels have to be added by maintainers after the PR/issue is created. I expect that when a maintainer gets to the PR/issue to triage it and adds a label, that a slack notification also gets sent to notify the rest of the team.

[zyqxd]

We also use this method in our project. It would be great to have this feature.

[mariskapansegrauw]

I second that, this will be very helpful.

[adamochayon]

The lack of this feature is currently blocking us from using the slack integration.
We work in a monorepo, and have a bot that auto-adds labels based on the team and directory.
Since that bot runs after the PR is opened (a couple of seconds), the notification isnt sent.

I have also tried adding the label upon opening the PR, using the gh cli by running gh pr create -f -l "ready-for-review", but it is still not sending a notification in this case.

[otoyo]

I wrote the patch for this issue in #1058.
Please help it to merge 🙌

[GeekPress]

Up for this one, it will be really helpful to be integrated ;)