Add external reference to export of attendees' data

[SilentResonance]

Is your feature request related to a problem? Please describe.
I'm importing from an external questionaire software via CSV. The questionaire creates a reference column, which is used at import to alf.io. After the check-in is finished we need to have the given reference at the export to match the persons for out tournament software.

Describe the solution you'd like
While looking through the code I found the part, where the query is created in service.js and downloadAllTicketsCSV() in EventApiController.java. And in there, the exportLines() function creates the field list to be exported.
Furthermore, I found out, that a reference column is specifically added, with the attendee CSV import.

What I would like to have is a checkbox to tick, when a reference column exists and its contents exported to CSV

Describe alternatives you've considered
Implementing it myself, but I find it hard how to setup a development environment for alf.io which directly replaces the docker instance.
I want to keep it dockerized instead of native installation in order to keep my OS clean.
Any hints and help are appreciated.

Additional context

• Docker container: alfio/alf.io:latest
• Alf.io version: v.2.0-M5