eCamp API access for 3rd parties

Currently our API is only accessible to the frontend client. We want to open up the API for 3rd party clients so that existing applications in the ecosystem can integrate with eCamp.

Examples of possible integrations:
- Qualix https://qualix.flamberg.ch
- eMeal https://emeal.zh11.ch
- cevi map tool https://map.cevi.tools
- cevi onlinemat tool https://github.com/cevi/onlinemat
- Kursadministration (Orbit) https://workflows.scout.ch
- Externer Kurs-Grobprogramm-Renderer https://github.com/twombli-rgb/digitales-kursprogramm
- Hering https://github.com/scout-ch/hering/issues/7

depends on https://github.com/ecamp/ecamp3/issues/1999