[GH-463] fixes to levels with costs

[ncontinanza]

Motivation

Part of lambdaclass/afk_gacha_game#463

Summary of changes

• Preload the attempt_cost currency of a level before sending the campaign to the client.
• Fix the index of the slot of the dungeon levels units.
• Add max_units of a level to the proto message

How to test it?

Start the client with this branch: lambdaclass/afk_gacha_game#477
You should be able to spend up to 5 Supplies playing dungeon levels, and see the cost of each level in the line up scene.
You shouldn't be able to play a dungeon level if you exceed the units limit.

Checklist

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

[agustinesco]

LGTM!