Add endpoint to store configuration data for the onsite UI

[RakanHowls]

The on-site software has a relatively small number of parameters that change every year:
https://github.com/eurofurence/reg-onsite-ui/blob/main/onsite/app/setupEFIteration.ts
(as well as some other, slowly changes pieces of information...)

If there was some way to store that data (which is less than 10kb) at some endpoint, we could completely separate code changes from configuration changes and keep the configuration in the reg database.

[RakanHowls]

It would be good if this endpoint could support ApiKeyAuth - we might have some use case of pulling some numbers (less than 1kb of data) from a different, highly protected system and making it available to the sponsor desk in particular at regular intervals.