*.ok files should allow course-specific data

[cycomachead]

"be liberal with what you accept and conservative with what you send..."

Would be very handy to have a way to augment an assignment's OK config with custom metadata.
This is useful for possible in class experiments, or passing data to other tools like gradescope autograders.

I think it's relatively fine to just accept anything and ignore unknown keys in the file. If that seems too messy, then I'd suggest adding a key called custom or metadata which is designed to be ignored and unvalidated -- and thus can take on any shape.