[GH-431] Add currency cost to levels

[ncontinanza]

Closes: https://github.com/lambdaclass/champions_of_mirra/issues/431

Associated PR: https://github.com/lambdaclass/champions_of_mirra/issues/463

Motivation

We need to be able to charge the user with a currency cost in order to get access to fight some of the levels. For example, the Dungeon levels will have a cost in Shovels.

Summary of changes

• Add an embedded array of CurrencyCost to the Level schema.
• Before starting a battle, check that the user can afford it.
• Substract the level's cost in currency when fighting the battle.

How to test it?

Run the backend and start the client in the main branch. For starters, everything should be working as usual, when none of the existing levels have a CurrencyCost associated.

Add this line to the map used for level creation in seeds.ex: currency_costs: [%CurrencyCost{currency_id: gold_currency.id, amount: 13}],
Also, you'll need to add this alias to that file: alias GameBackend.Users.Currencies.CurrencyCost

You should end up with something like this:

levels =
  Enum.flat_map(Enum.with_index(rules, 1), fn {campaign_rules, campaign_index} ->
    {_, campaign} =
      Campaigns.insert_campaign(
        %{
          game_id: champions_of_mirra_id,
          super_campaign_id: super_campaign.id,
          campaign_number: campaign_index
        },
        returning: true
      )

    Enum.map(1..campaign_rules.length, fn level_index ->
      %{
        game_id: champions_of_mirra_id,
        campaign_id: campaign.id,
        level_number: level_index,
        experience_reward: 100 * level_index,
        attempt_cost: [%CurrencyCost{currency_id: gold_currency.id, amount: 13}],
        inserted_at: NaiveDateTime.utc_now() |> NaiveDateTime.truncate(:second),
        updated_at: NaiveDateTime.utc_now() |> NaiveDateTime.truncate(:second)
      }
    end)
  end)

Now all the levels cost 13 Gold. Before fighting a new level, check your current amount of gold. If you fight the level and lose the battle, your amount of gold should have decreased by 13. If you won, your gold amount should now be previous amount + level reward - level cost.

Checklist

• Tested the changes locally.
• Reviewed the changes on GitHub, line by line.
• This change requires new documentation.
  • Documentation has been added/updated.

[lotuuu]

Just as a nitpicky comment, I like the uncountable version of cost better for the cost of playing a level. Instead of attempt_costs, i'd go for attempt_cost.

Also, the description is now outdated as it uses the previous "currency_costs" attribute.

Lastly, is there an issue already in the client for implementing handling of "cant afford" errors? Because of course right now this breaks in the client