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
"run_as": [],
              "metadata": {},
              "transient_metadata": {
                "enabled": true
              }
            }
          }""";
    private static final String ELASTIC_ENTERPRISE_SEARCH_SERVER_ROLE_DESCRIPTOR = """
        {
            "cluster": [
                "manage",
                "manage_security"
            ],
            "indices": [
                {
                    "names": [
                        "search-*",
                        ".elastic-analytics-collections",
                        ".ent-search-*",
                        ".monitoring-ent-search-*",
                        "metricbeat-ent-search-*",
                        "enterprise-search-*",
                        "logs-app_search.analytics-default",
                        "logs-elastic_analytics.events-*",
                        "logs-enterprise_search.api-default",
                        "logs-enterprise_search.audit-default",
                        "logs-app_search.search_relevance_suggestions-default",
                        "logs-crawler-default",
                        "logs-elastic_crawler-default",
                        "logs-workplace_search.analytics-default",