Implement GetDungeonAfkRewards message

[ncontinanza]

Motivation

We need to get the Dungeon AFK rewards and send them to the client.

Summary of changes

Implement the GetDungeonAfkRewards message and the logic for its response.

How to test it?

Follow the steps to claim the Dungeon AFK Rewards here: lambdaclass/afk_gacha_game#487

Checklist

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

[lotuuu]

Let's add some simple testing of the GetDungeonAfkRewards message with the SocketTester! even if really basic these tests are the most important ones since they can easily be broken by missing preloads or schema modifications

[ncontinanza]

We won't be using this message as the Dungeon AFK rewards will be claimed automatically.
In the future, this issue will have to be addressed for that behaviour to work properly: lambdaclass/afk_gacha_game#491