Registers a mounted workshop addon as a SlashCo addon by its workshop ID, adding it to SlashCo.GetAddons and, Server-side, queuing it for client download via resource.AddWorkshop.
This normally isn't required, as any addon containing a lua/slashco folder is auto-detected by SlashCo.FindSlashCoAddons. Addons that don't match that check can call this from the SlashCo:RegisterAddons hook instead.