Update /api/games to allow searching by provider_games.slug

[rkr87]

Sorry if this isn't the right repo to raise this in, I couldn't find one specific to the REST API.

I'd like to convert between store IDs, eg, something like;

https://lutris.net/api/games?service=gog&slug=2004253604

Would return Into the Breach where I could then easily access all of the other providers of that game and their corresponding store IDs.

I realise I can kind of achieve this by searching using the game's name but that has the potential for multiple results, whereas search by the ID would be guaranteed to return the specific game I'm interested in.

Thanks.