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/main/java/org/elasticsearch/xpack/fleet/Fleet.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.ExceptionsHelper;
import org.elasticsearch.ResourceNotFoundException;
import org.elasticsearch.Version;
import org.elasticsearch.action.ActionListener;
import org.elasticsearch.action.ActionRequest;
import org.elasticsearch.action.ActionResponse;
import org.elasticsearch.action.admin.cluster.snapshots.features.ResetFeatureStateResponse.ResetFeatureStateStatus;
import org.elasticsearch.action.admin.indices.template.put.PutIndexTemplateRequest;
import org.elasticsearch.action.datastreams.DeleteDataStreamAction;
import org.elasticsearch.action.datastreams.DeleteDataStreamAction.Request;
import org.elasticsearch.action.support.IndicesOptions;
import org.elasticsearch.action.support.IndicesOptions.Option;
import org.elasticsearch.client.internal.Client;
import org.elasticsearch.cluster.metadata.ComposableIndexTemplate;
import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;