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/src/main/java/org/elasticsearch/xpack/security/authc/service/ElasticServiceAccounts.java
"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",
                        "logs-workplace_search.content_events-default",
                        ".elastic-connectors*"
                    )
                    .privileges("manage", "read", "write")
                    .build() },
            null,
            null,
            null,
            null,
            null
        )
    );
    private static final ServiceAccount FLEET_ACCOUNT = new ElasticServiceAccount(
        "fleet-server",
        new RoleDescriptor(
            NAMESPACE + "/fleet-server",
            new String[] { "monitor", "manage_own_api_key" },
            new RoleDescriptor.IndicesPrivileges[] {
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(