Add pipeline schedules subscription

What does this MR do and why?

Adds the ciPipelineScheduleStatusUpdated subscription where users/clients can subscribe to a project and get realtime updates on any pipeline schedule, if the last pipeline status changes. The subscription returns single schedule objects at a time, if the last pipeline status was updated.

References

Screenshots or screen recordings

How to set up and validate locally

1. Enable FF ci_pipeline_schedules_status_realtime
2. Create a pipeline schedule (or multiple to help with testing)
3. Visit http://gdk.test:3000/-/graphql-explorer and use the subscription with your project ID

subscription {
  ciPipelineScheduleStatusUpdated(projectId: "gid://gitlab/Project/19") {
    id
    lastPipeline {
      detailedStatus {
        id
      }
    }
  }
}

4. Trigger the schedule and watch data update in the explorer and network tab (sockets)

MR acceptance checklist

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Related to #560596 (closed)