Request schema extensions upstream

[hbruch]

The ParkAPI city.json schema currently does neither support opening_hours nor fees. We should ask them to add these to the schema. Additionally, I'd like to add a new attribute to provide additional notices to customers, like e.g. upcoming events/opening hour changes etc.

[hbruch]

Done for opening_hours and fee_hours via offenesdresden/ParkAPI#216

Still required: an attribute (e.g. "notes" or "customer_notice") to provide additional infos to customers (e.g. Upcoming changes or the like)