What is the bug?
Search API for rules does not support returning specific fields when the _source field is specified as part of the request body.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Make following search REST API call

POST /_plugins/_security_analytics/rules/_search
{
  "query": {
    "nested": {
      "path": "rule",
      "query": {
        "term": {
          "rule.category": {
            "value": "network"
          }
        }
      }
    }
  },
  "_source": ["rule.title"]
}

It should only return the title field

{
  ...
    "hits": [
      {
        "_index": ".opensearch-sap-pre-packaged-rules-config",
        "_id": "503fe26e-b5f2-4944-a126-eab405cc06e5",
        "_score": 3.9827108,
        "_source": {
          "title": "Kerberos Network Traffic RC4 Ticket Encryption"
        }
      },
     ...
    ]
  }
}

but it returns all the fields as part of source

What is the expected behavior?
Only specified fields should be returned

What is your host/environment?
Opensearch 2.5

Do you have any screenshots?
NA

Do you have any additional context?
The underlying index does support this, so if the call is made against .opensearch-sap-pre-packaged-rules-config index, it works correctly