Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/security/qa/service-account/src/javaRestTest/java/org/elasticsearch/xpack/security/authc/service/ServiceAccountIT.java
"logs-workplace_search.content_events-default",
                        ".elastic-connectors*"
                    ],
                    "privileges": [
                        "manage",
                        "read",
                        "write"
                    ],
                    "allow_restricted_indices": false
                }
            ],
            "applications": [],
            "run_as": [],
            "metadata": {},
            "transient_metadata": {
                "enabled": true
            }
        }""";
    @ClassRule
    public static ElasticsearchCluster cluster = ElasticsearchCluster.local()
        .nodes(2)
        .module("analysis-common")
        .module("rest-root")
        .setting("xpack.license.self_generated.type", "trial")
        .setting("xpack.security.enabled", "true")
        .setting("xpack.security.authc.token.enabled", "true")
        .setting("xpack.security.authc.api_key.enabled", "true")
        .setting("xpack.security.http.ssl.enabled", "true")
        .setting("xpack.security.http.ssl.certificate", "node.crt")
        .setting("xpack.security.http.ssl.key", "node.key")