Option to Pin CI failure messages in Slack channel

[stop-cran]

What feature do you want to see added?

I faced a problem using Slack notifications for aggregated channels - CI failure message sometimes gets wiped by other messages, so it's not clear at first glance whether there are any ongoing CI failures, or everything is okay at the moment.

I would suggest to add an option to the Slack plugin to pin CI failure/unstable message to the target Slack channel and unpin it once the same CI is back to normal. Also do not pin repetitive failure messages to keep the pinned message list prompt.

This option should make checking for ongoing failures really quick even for big aggregated Slack channels - just check the list of pinned messages, which should be short.

Upstream changes

No response