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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.core.security.authz.store;
import org.elasticsearch.action.ActionListener;
import org.elasticsearch.action.admin.cluster.remote.RemoteInfoAction;
import org.elasticsearch.action.admin.cluster.repositories.get.GetRepositoriesAction;
import org.elasticsearch.action.admin.indices.alias.IndicesAliasesAction;
import org.elasticsearch.action.admin.indices.delete.DeleteIndexAction;
import org.elasticsearch.action.admin.indices.mapping.put.PutMappingAction;
import org.elasticsearch.action.admin.indices.rollover.RolloverAction;
import org.elasticsearch.action.admin.indices.settings.put.UpdateSettingsAction;