Import and export proposal images and attachements

Is your feature request related to a problem? Please describe.
Currently, Decidim lacks a straightforward way to import and export proposals along with their associated attachments and images. This limitation makes it challenging for admins to import proposals for usage on the platform or to export them to reuse the data in other environments. The problem is so present, I even did a little Gradio space that gets all the images from the API but this is definitely not a long term solution: https://huggingface.co/spaces/monsimas/exporter-les-images

Describe the solution you'd like
Extend the import and export functionality of Decidim to support the inclusion of attachments and images alongside proposals. The best approach (debatable) seems to be to use a CSV file format with dedicated columns for attachment and image URLs. This is how some very big Drupal (Feeds) and Wordpress (WP all import) modules manage it.

Here's how it could work:

For exports - Decidim would generate a CSV or other export file that includes all the traditional columns for proposal data and additionally, attachment URLs, and image URLs.

For imports: admins would provide a CSV or other supported Decidim import file that follows the same structure, with columns for attachment and image URLs. The platform would parse the CSV file, create the proposals, and download the attachments and images from the specified URLs, associating them with the respective proposals.

Describe alternatives you've considered
ZIP file upload and download.

Funded by
Currently, there is no specific funding available for this feature request. However, we could maybe co-finance it if another organisation would be interested in sponsoring.