[Response Ops][Cases] Define a better type for useFetchAlertData hooks

[jonathan-buttner]

The useFetchAlertData hooks defined for security solution and observability will return Record<string, unknown> when this PR is merged: #123883

Ideally we could help the recipient by returning a more definitive type. The issue is that security solution's implementation recursively breaks the results up into a nested object here:
https://github.com/elastic/kibana/blob/main/x-pack/plugins/security_solution/public/cases/pages/use_fetch_alert_data.ts#L30
https://github.com/elastic/kibana/blob/main/x-pack/plugins/security_solution/public/common/utils/alerts.ts#L128

where as the observability function does not. It simply returns the data the way the alerts client sends it which looks more like:

Alerts client response

{
  "took": 2,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "max_score": null,
    "hits": [
      {
        "_index": ".internal.alerts-observability.metrics.alerts-default-000001",
        "_id": "662aba44-d24f-47f9-8933-0e0f81cae6df",
        "_seq_no": 696,
        "_primary_term": 1,
        "_score": null,
        "_source": {
          "kibana.alert.rule.category": "Metric threshold",
          "kibana.alert.rule.consumer": "infrastructure",
          "kibana.alert.rule.execution.uuid": "40cc6f59-1101-45b1-81df-fdb47f87fcc4",
          "kibana.alert.rule.name": "Jon rule",
          "kibana.alert.rule.producer": "infrastructure",
          "kibana.alert.rule.rule_type_id": "metrics.alert.threshold",
          "kibana.alert.rule.uuid": "325830a0-7eba-11ec-a3b9-350ed5c64d7e",
          "kibana.space_ids": [
            "default"
          ],
          "kibana.alert.rule.tags": [],
          "@timestamp": "2022-01-26T22:37:20.409Z",
          "kibana.alert.reason": "Document count has reported no data over the past 1m for *",
          "kibana.alert.duration.us": 26767233000,
          "kibana.alert.instance.id": "*",
          "kibana.alert.start": "2022-01-26T15:11:13.176Z",
          "kibana.alert.uuid": "662aba44-d24f-47f9-8933-0e0f81cae6df",
          "kibana.alert.status": "active",
          "kibana.alert.workflow_status": "open",
          "event.kind": "signal",
          "event.action": "active",
          "kibana.version": "8.1.0",
          "tags": []
        },
        "fields": {
          "kibana.alert.rule.rule_type_id": [
            "metrics.alert.threshold"
          ],
          "kibana.alert.workflow_status": [
            "open"
          ],
          "kibana.space_ids": [
            "default"
          ],
          "kibana.alert.rule.consumer": [
            "infrastructure"
          ]
        },
        "sort": [
          1643236640409
        ]
      }
    ]
  }
}

Observability formatted result

{
  "<_id value>": {
    _id: <value>,
    _index: <value>
    ... <contents of the _source field spread here>
  }
}

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)