Changes at "Export budget votes"

Is your feature request related to a problem? Please describe.
I'm a computer science researcher working on voting rules for participatory budgeting, in particular the Method of Equal Shares which ensures proportional representation in the budgeting outcome. This method has been used by three cities in 2023, and more cities have expressed interest in using the method. To compute which projects are winning, the method needs more than just the vote count for the project - it needs to know for each voter, the set of projects they voted for.

Decidim currently does not allow to export this data from the admin interface (the budget export currently only gives the vote count, not disaggregated votes). This makes it tricky for a city to provide us with data from their prior budgets so we can simulate what our method would have done. It also makes it tricky for a city to use decidim if it wants to use custom voting methods. Finally, cities using decidim cannot easily contribute detailed voting data to the pabulib library of participatory budgeting datasets.

Describe the solution you'd like
An additional export button that produces a file with full vote data. Many different export formats would be okay. The closest to the current export would add an additional column \"voters\" to each project row, which would be a list of voter ids that voted for the project. A more involved solution would be to provide an export in the pabulib .pb file format.

Describe alternatives you've considered
The alternative would be to use direct database queries that would need to be performed by an admin with access to the server. We could help with that by providing explicit queries or scripts that do that.

Does this issue could impact on users private data?
A privacy-respecting implementation would not use users' internal ID numbers in the export but would assign new random user IDs at export time. If internal user IDs are used, the vote could potentially be connected to the real name of the user.

With this procedure, there should not be an impact on users' private data.

Funding
I don't have funding specifically, but I could ask around in my academic community. We may have people/students experienced with ruby on rails that could implement this.