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
"name": "_service_account",
            "type": "_service_account"
          },
          "lookup_realm": {
            "name": "_service_account",
            "type": "_service_account"
          },
          "authentication_type": "token"
        }
        """;
    private static final String ELASTIC_FLEET_SERVER_ROLE_DESCRIPTOR = """
        {
              "cluster": [
                "monitor",
                "manage_own_api_key"
              ],
              "indices": [
                {
                  "names": [
                    "logs-*",
                    "metrics-*",
                    "traces-*",
                    ".logs-endpoint.diagnostic.collection-*",
                    ".logs-endpoint.action.responses-*"
                  ],
                  "privileges": [
                    "write",
                    "create_index",
                    "auto_configure"
                  ],