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/fleet/src/test/java/org/elasticsearch/xpack/fleet/FleetTests.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.fleet;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.indices.SystemIndexDescriptor;
import org.elasticsearch.indices.SystemIndices;
import org.elasticsearch.indices.SystemIndices.Feature;
import org.elasticsearch.test.ESTestCase;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;
import static org.hamcrest.Matchers.containsInAnyOrder;
public class FleetTests extends ESTestCase {