Implement Rewards for Completed Challenges

[Razano26]

Introduce rewards for players who complete challenges, and store information about rewards given in MongoDB.

Tasks:

• Define reward types in the challenges collection (e.g., in-game items, experience).
• Update the player_progress or completed_challenges collection with the reward given.
• Implement functionality to deliver the reward to the player once the challenge is marked as completed.
• Test the reward system by completing challenges and ensuring players receive the correct rewards.