Draft: Add stages subscription experiment

What does this MR do and why?

Add stages subscription experiment. Can be tested E2E with POC fullstack MR here: !212825

References

Screenshots or screen recordings

stage-subscription-demo

How to set up and validate locally

1. Test in explorer with

subscription stageJobsUpdated($stageId: CiStageID!) {
  ciStageUpdated(stageId: $stageId) {
    id
    detailedStatus {
      id
      action {
        id
        confirmationMessage
        icon
        path
        title
      }
      detailsPath
      deploymentDetailsPath
      group
      hasDetails
      icon
      tooltip
    }
    name
    scheduled
    scheduledAt
  }
}

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #580125