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
/*
 * 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.security.authc.service;
import org.elasticsearch.common.Strings;
import org.elasticsearch.xpack.core.security.authz.RoleDescriptor;
import org.elasticsearch.xpack.core.security.authz.store.ReservedRolesStore;
import org.elasticsearch.xpack.core.security.user.User;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.function.Function;
import java.util.stream.Collectors;
final class ElasticServiceAccounts {
    static final String NAMESPACE = "elastic";
    private static final ServiceAccount ENTERPRISE_SEARCH_ACCOUNT = new ElasticServiceAccount(
        "enterprise-search-server",
        new RoleDescriptor(
            NAMESPACE + "/enterprise-search-server",
            new String[] { "manage", "manage_security" },
            new RoleDescriptor.IndicesPrivileges[] {
                RoleDescriptor.IndicesPrivileges.builder()
                    .indices(
                        "search-*",
                        ".elastic-analytics-collections",
                        ".ent-search-*",
                        ".monitoring-ent-search-*",