Make get_protected_columns() S3 to avoid hard-coding things

[nikosbosse]

          this is another pain point. I guess we could have a s3 method for all classes that lists the protected columns for that ? Are we using this internally anywhere we don't know about the class of the input the user has made? If no we could do this and make this a lot more modular/hard coded

Originally posted by @seabbs in #837 (comment)

[nikosbosse]

I'm voting to move this scoringutils version 2.1 (and prioritise it then). It's fair to say that we missed our "first quarter of 2024" deadline by a bit and I'd like to get a first version on CRAN asap

[seabbs]

Agree though it does block adding more forecast classes in a nice way. If I get time I will look at it