Add column to Argo structural metadata spreadsheet to support new language attribute for files

[andrewjbtw]

We've added a language field to the Cocina file model, initially as a way to support captioning work. This will allow us to show users which language a particular file uses. See sul-dlss/cocina-models#640

Now we need to make this field user-editable. To do this in Argo, we need to add the field to the structural metadata spreadsheet. We could call it file_language and it should be optional.

The spreadsheet is used in the following Argo features:

• item show page (in the contents section)
  • download CSV
  • upload CSV
• bulk actions
  • export structural metadata
  • import structural metadata

Ideally, we would also be able to display this in the UI in the contents grid but I'll defer that to another ticket.