Create timeline events from labels

What does this MR do and why?

Create an incident timeline event every time incident's labels are updated.

Related to #365489 (closed)

The feature is behind an incident_timeline_events_from_labels feature flag [Feature flag] Rollout of `incident_timeline_ev... (#369416)

Screenshots or screen recordings

System notes

Timeline event

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Ensure incident_timeline_events_from_labels feature flag is enabled (Feature.enable(:incident_timeline_events_from_labels))
2. Navigate to Monitor > Incidents page
3. Create or open an existing incident
4. Add and/or remove some labels
5. Check Timeline tab to see the result

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.