feat: Subscribe to decision evaluation updates

[saig0]

Description

Add a new subscription to the GraphQL API for decision evaluations.

---

Subscribe to all decision evaluations:

subscription {
  decisionEvaluationUpdates {
    key
    decision {
      key
      decisionId
    }
    decisionOutput
    state
  }
}

Response message:

{
  "data": {
    "decisionEvaluationUpdates": {
      "key": "2251799813685390",
      "decision": {
        "key": "2251799813685277",
        "decisionId": "decision_a"
      },
      "decisionOutput": "\"A+\"",
      "state": "EVALUATED"
    }
  }
}

---

Related to #286