Add `incident_timeline` feature flag

What does this MR do and why?

Adds the incident_timeline feature flag.

Will be used for the development of the Incident Timelines epic.

Screenshots or screen recordings

No visual change.

How to set up and validate locally

N/A

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.