Add ability to sort by escalation status to issues GraphQL query

What does this MR do and why?

Related issue: #332056 (closed)

This adds the ability to sort by escalation status in GraphQL, for usage from the incident list (project > Monitor > Incidents).

The escalation status feature for incidents is still in progress at the moment. I've opted not to include a feature flag check, because it's likely to be enabled very soon. This should correspondingly merge after feature flag removal, though it's not strictly blocking. !80444 (merged) Rollout has it a few lil' snags, so I've opted to add in feature flag checks in this MR. This can now be merged whenever.

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:incident_escalations)

2. Create a new incident, and update the status via rails console

   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)

3. Open graphql explorer /-/graphql-explorer & run query

   query issues {
     project(fullPath: "root/my_project") {
       issues(types: INCIDENT, sort: ESCALATION_STATUS_DESC) {
         nodes {
           iid
           title
           type
           escalationStatus
         }
       }
     }
   }

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