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
}
    }
    private String loadTemplateSource(String resource) {
        return TemplateUtils.loadTemplate(resource, Version.CURRENT.toString(), MAPPING_VERSION_VARIABLE);
    }
    @Override
    public List<ActionHandler<? extends ActionRequest, ? extends ActionResponse>> getActions() {
        return Arrays.asList(
            new ActionHandler<>(GetGlobalCheckpointsAction.INSTANCE, GetGlobalCheckpointsAction.TransportAction.class),
            new ActionHandler<>(GetGlobalCheckpointsShardAction.INSTANCE, GetGlobalCheckpointsShardAction.TransportAction.class)
        );
    }
    @Override
    public List<RestHandler> getRestHandlers(
        Settings settings,
        RestController restController,
        ClusterSettings clusterSettings,
        IndexScopedSettings indexScopedSettings,
        SettingsFilter settingsFilter,
        IndexNameExpressionResolver indexNameExpressionResolver,
        Supplier<DiscoveryNodes> nodesInCluster
    ) {
        return Arrays.asList(
            new RestGetGlobalCheckpointsAction(),
            new RestFleetSearchAction(restController.getSearchUsageHolder()),
            new RestFleetMultiSearchAction(settings, restController.getSearchUsageHolder())
        );
    }
}