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
.map(SystemDataStreamDescriptor::getDataStreamName)
                        .collect(Collectors.toList())
                        .toArray(Strings.EMPTY_ARRAY)
                );
                EnumSet<Option> options = request.indicesOptions().options();
                options.add(Option.IGNORE_UNAVAILABLE);
                options.add(Option.ALLOW_NO_INDICES);
                request.indicesOptions(new IndicesOptions(options, request.indicesOptions().expandWildcards()));
                client.execute(
                    DeleteDataStreamAction.INSTANCE,
                    request,
                    ActionListener.wrap(response -> SystemIndexPlugin.super.cleanUpFeature(clusterService, client, listener), e -> {
                        Throwable unwrapped = ExceptionsHelper.unwrapCause(e);
                        if (unwrapped instanceof ResourceNotFoundException) {
                            SystemIndexPlugin.super.cleanUpFeature(clusterService, client, listener);
                        } else {
                            listener.onFailure(e);
                        }
                    })
                );
            } catch (Exception e) {
                Throwable unwrapped = ExceptionsHelper.unwrapCause(e);
                if (unwrapped instanceof ResourceNotFoundException) {
                    SystemIndexPlugin.super.cleanUpFeature(clusterService, client, listener);
                } else {
                    listener.onFailure(e);
                }
            }
        } else {
            SystemIndexPlugin.super.cleanUpFeature(clusterService, client, listener);