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/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java
import org.elasticsearch.common.collect.MapBuilder;
import org.elasticsearch.transport.TcpTransport;
import org.elasticsearch.xpack.core.ilm.action.GetLifecycleAction;
import org.elasticsearch.xpack.core.ilm.action.PutLifecycleAction;
import org.elasticsearch.xpack.core.monitoring.action.MonitoringBulkAction;
import org.elasticsearch.xpack.core.security.action.apikey.InvalidateApiKeyAction;
import org.elasticsearch.xpack.core.security.action.privilege.GetBuiltinPrivilegesAction;
import org.elasticsearch.xpack.core.security.action.profile.ActivateProfileAction;
import org.elasticsearch.xpack.core.security.action.profile.GetProfilesAction;
import org.elasticsearch.xpack.core.security.action.profile.SuggestProfilesAction;
import org.elasticsearch.xpack.core.security.action.user.ProfileHasPrivilegesAction;
import org.elasticsearch.xpack.core.security.authz.RoleDescriptor;
import org.elasticsearch.xpack.core.security.authz.privilege.ConfigurableClusterPrivilege;
import org.elasticsearch.xpack.core.security.authz.privilege.ConfigurableClusterPrivileges.ManageApplicationPrivileges;
import org.elasticsearch.xpack.core.security.authz.privilege.ConfigurableClusterPrivileges.WriteProfileDataPrivileges;
import org.elasticsearch.xpack.core.security.support.MetadataUtils;
import org.elasticsearch.xpack.core.security.user.KibanaSystemUser;
import org.elasticsearch.xpack.core.security.user.UsernamesField;
import org.elasticsearch.xpack.core.transform.transforms.persistence.TransformInternalIndexConstants;
import org.elasticsearch.xpack.core.watcher.execution.TriggeredWatchStoreField;
import org.elasticsearch.xpack.core.watcher.history.HistoryStoreField;
import org.elasticsearch.xpack.core.watcher.watch.Watch;
import java.util.Collection;
import java.util.Collections;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.stream.Collectors;
public class ReservedRolesStore implements BiConsumer<Set<String>, ActionListener<RoleRetrievalResult>> {