Add incident attribute to slack notif settings

What does this MR do and why?

This MR adds the new setting option mentioned in #367550 (closed)

The idea, for now, is to receive the incident updates in a new specific channel along with the existing channel for general issue updates.

So, if one enables this setting, they will receive updates for incidents in both slack channels mentioned under issue and incident.

Check the below attached video for a demo.

Screenshots or screen recordings

slack notification demo

How to set up and validate locally

1. Check integration settings by going to Settings > Integrations.
2. Select Slack notifications and follow the steps mentioned in Slack notification integration doc.
3. Try creating, closing, and re-opening incidents.
4. Check notifications in the respective slack channels that are configured with the integration.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #367550 (closed)