For bod layers, we get the attributations from the layers service (gaLayers), for others directly from the (ol) layer object.

To be more consistent, we could also set the attribution property on the olLayer object for bod layers (to be done in this function: https://github.com/geoadmin/mf-geoadmin3/blob/9952f921921180d1893d6b813089aa569a35be30/src/components/map/MapService.js#L519)...it's duplicated information, but it's more consistent as we do it also for opacity, label, etc already.