Add escalation-related attributes to issue webhook

What does this MR do and why?

• Related issue: #330284 (closed) (webhook details)
• Docs changes: !80441 (merged)

This MR adds support for escalation_status and escalation_policy attributes to the issue webhook payload.

Both fields correspond to incident-only features, so they'll only appear in the webhook payload for incidents. escalation_policy is only available for GitLab Premium, so the field will only appear correspondingly.

As this is still behind the incident_escalations feature flag, I've opted to exclude a changelog. If the incident_escalations flag is rolled out before this merges, I'll add the changelog trailers then.

Relevant sample hook output

Updating status	Updating policy	Updating status & policy	Updating unrelated field (milestone)

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:incident_escalations)

2. Create or navigate to a GitLab Premium project
3. Create a new incident (New Issue w/ type=Incident || Monitor > Incidents)
4. If not yet created, create an escalation policy under Monitor > Escalation Policies (any rules are fine)
5. Setup a webhook which pushes Issues events at Settings > Webhooks
   • Possible pre-req: Setup a webhook receiver to view output (I can't remember if this is required for initial setup...)
   • Navigate to the Edit page of the hook to view hook output
6. Update the status of the incident in the rails console (dropdown coming in !66165 (merged)) & view the hook output

   project = Project.find(<PROJECT_ID>)
   incident = project.issues.last
   ::Issues::UpdateService.new(project: project, current_user: User.first, params: { escalation_status: { status: :acknowledged } }).execute(incident)

   • escalation_status should be an attribute in the body with value of Acknowledged
   • escalation_policy should be an attribute in the body with value of nil
   • escalation_status should be present in the changes section of the payload
7. Update the policy of the incident in the rails console (dropdown coming in !66165 (merged)) & view the hook output

   project = Project.find(<PROJECT_ID>)
   incident = project.issues.last
   policy = project.incident_management_escalation_policies.last
   ::Issues::UpdateService.new(project: project, current_user: User.first, params: { escalation_status: { policy: policy } }).execute(incident)

   • escalation_status should be an attribute in the body with value of Triggered (setting a policy re-triggers an incident)
   • escalation_policy should be an attribute in the body with value of { id: <ID>, name: <name> }
   • escalation_status & escalation_policy should both be present in the changes section of the payload
8. Unset the policy & view the hook output

   project = Project.find(<PROJECT_ID>)
   incident = project.issues.last
   ::Issues::UpdateService.new(project: project, current_user: User.first, params: { escalation_status: { policy: nil } }).execute(incident)

   • only escalation_policy should be present in the changes section of the payload
9. Change some other attribute via the UI to see a payload without escalation fields

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.