Allow control of fieldnames included in JSON and CSV exports/imports

I just mean that the title of this thread doesn’t mention imports at all, so our little 3-person discussion so far may be missing interested voices.

Meanwhile, though, we should make sure to participate in the refactoring of imports (and potentially exports as well?) in 5.4: [TW2036] Planning for v5.4.0 - #47 by saqimtiaz