Allow downloadable variants

[bandieramonte]

Is your feature request related to a problem? Please describe.
When adding books to the shop, I wish to add the digital and paperback variants or formats of each book. As of now, Smartstore allows a product to be classified as digital, so I can add a downloadable document. However, this is not possible for a certain variant within a product. In other words, either the whole product is downloadable or not, but it is not possible to set a specific variant of a product as downloadable.

Describe the solution you'd like
To make is possible to mark a variant of a product as downloadable, besides also allowing to mark the whole product as downloadable. This way an admin can create a product (book), set a variant (PDF for instance) as downloadable defining its file to download, and also set a physical variant, for instance, paperback, which is not downloadable. Then, a customer can go to the books details page, and choose among the book formats from a dropdown list, radio button list or other presentation. If it chooses the pdf format, then it is downloadable and not shippable. If it selects paperback, then it must be treated as shippable without any file to download.

Describe alternatives you've considered
After discussing this with Smartstore's internal team, an alternative solution they recommended was using product groups. The admin would create multiple products referring to the same book, one per each format, and group them together. This results however in added work for admins creating the same book multiple times, only the format would be different. Also, for users, it is not the same to see product groups than a product where he or she can choose the format within the same page. Grouped products in this scenario seem rather hacky.

Additional context
This is an example of what the ideal solution could look like:

There it is possible in the same product details page, to choose between kindle or paperback. Of course, kindle might not be one of the formats available, it could rather be PDF, Mobi, EPUB, etc. The selection can be a dropdown list as well.