Sitemap service for map.geo.admin.ch

https://github.com/geoadmin/mf-chsdi3/issues/849

This will replace current static sitemaps in map.geo.admin.ch. Note that another PR in ga3 is needed to use this service.

But it doesn't hurt to review/merge this now.
Why not access the layers listing directly through a DB access to layersconfig table, filter by staging and topic value. (those are correct)

You'd have all the info in one request to the DB. Here you're performing one sub request to `topics` then X requests to `CatalogServer`, where X is the number of topics you found. Code would probably look much more simple also.