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.xpack.fleet.action.GetGlobalCheckpointsShardAction;
import org.elasticsearch.xpack.fleet.rest.RestFleetMultiSearchAction;
import org.elasticsearch.xpack.fleet.rest.RestFleetSearchAction;
import org.elasticsearch.xpack.fleet.rest.RestGetGlobalCheckpointsAction;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.util.Arrays;
import java.util.Collection;
import java.util.EnumSet;
import java.util.List;
import java.util.Map;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import static org.elasticsearch.xpack.core.ClientHelper.FLEET_ORIGIN;
/**
 * A plugin to manage and provide access to the system indices used by Fleet.
 *
 * Currently only exposes general-purpose APIs on {@code _fleet}-prefixed routes, to be more specialized as Fleet's requirements stabilize.
 */
public class Fleet extends Plugin implements SystemIndexPlugin {
    private static final int CURRENT_INDEX_VERSION = 7;
    private static final String VERSION_KEY = "version";
    private static final String MAPPING_VERSION_VARIABLE = "fleet.version";
    private static final List<String> ALLOWED_PRODUCTS = List.of("kibana", "fleet");
    @Override
    public Collection<Object> createComponents(
        Client client,
        ClusterService clusterService,
        ThreadPool threadPool,
        ResourceWatcherService resourceWatcherService,