[FEATURE] Allow specifying card variant when adding to collection

[totaltentigers]

Is your feature request related to a problem? Please describe.
I would like more granularity within the collection tracking feature. I'd like to be able to track cards based on their variant. This includes rarity like Holo and Reverse Holo version of a card, but also special cards like Prerelease Promos with the set logo overlaying the card art, Alternate art, and Pokemon League promos. AFAIK, these variants usually do not have different numbers than the standard version within their sets, and thus cannot be currently tracked.

Describe the solution you'd like
When adding a card to my collection, I would like the ability to select a variant for that card's instance in my collection. I would then like to be able to see which and how many variants I own for a specific card.

Describe alternatives you've considered
None.

Additional context
I doubt the data for these variants are being tracked anywhere accessible via an API. There likely is no way to show the user WHICH variants exist per card in all of the TCG. All variant options would have to be available for each card when adding to the collection, and so this level of granularity would probably have to be up to the user's discretion. I understand presenting these options to the user may not be in line with DeckBox's current very clean and simple UI/UX. Another TCG app I have used is Pokecardex (Android), which allows this granularity in its collection tracking feature, though that app presents a modal with options to the user when adding a card to the collection.

[r0adkll]

This was a feature that I briefly pondered upon but chose to side table it in favor of getting the collection feature completed and pushed out to my users. I know that the prerelease promos have their own number in the Black Star Promos set (like here: https://pokemontcg.io/cards/smp-SM158) although admittedly some are missing from that set.

I should be able to use the cardlist pdf's from pokemons official site to infer available variants of cards (i.e. https://assets.pokemon.com/assets/cms2/pdf/trading-card-game/checklist/sm11_web_cardlist_en.pdf). It looks like any non-gx that doesn't have a holo-rare rarity have standard and reverse holo variants, then holo-rare have holo and reverse holo variants. League or other like promos would probably have to be done by hand so I'm not sure how feasible that can be.

I'm currently in the middle of a design refresh so I will look into possible ways to introduce this granularity into the new design in a clean manner.