Improve custom report editor menu card exporters

We export menu card data in the report editor for selecting columns to add to a report, or choosing audience types:

The return structure for each is generated in helper methods (here and here), rather than in specific exporter classes. In order to make this code easier to extend/re-use (we want to do so in Workplace), move the implementation from helper methods into exporters, which is where it probably should have been in the first place

Deprecate original methods, as no longer required