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
import org.elasticsearch.cluster.node.DiscoveryNodes;
import org.elasticsearch.cluster.routing.allocation.AllocationService;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
import org.elasticsearch.common.settings.ClusterSettings;
import org.elasticsearch.common.settings.IndexScopedSettings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.settings.SettingsFilter;
import org.elasticsearch.env.Environment;
import org.elasticsearch.env.NodeEnvironment;
import org.elasticsearch.indices.ExecutorNames;
import org.elasticsearch.indices.SystemDataStreamDescriptor;
import org.elasticsearch.indices.SystemIndexDescriptor;
import org.elasticsearch.indices.SystemIndexDescriptor.Type;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.SystemIndexPlugin;
import org.elasticsearch.repositories.RepositoriesService;
import org.elasticsearch.rest.RestController;
import org.elasticsearch.rest.RestHandler;
import org.elasticsearch.script.ScriptService;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.tracing.Tracer;
import org.elasticsearch.watcher.ResourceWatcherService;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import org.elasticsearch.xcontent.XContentParser;
import org.elasticsearch.xcontent.XContentParserConfiguration;
import org.elasticsearch.xcontent.XContentType;
import org.elasticsearch.xpack.core.template.TemplateUtils;
import org.elasticsearch.xpack.fleet.action.GetGlobalCheckpointsAction;