Feat/fees refund

closes #104
closes #107

This pull request implements the fees refund feature and introduces several enhancements and fixes to the subscription management system. The key changes are as follows:

Summary

• Implements fee refund calculations and handling.
• Enhances subscription management with new events and improved error handling.
• Refactors fee calculation logic to support tiered and linear calculations.
• Adds comprehensive tests for new features and edge cases.

Description

New Features and Enhancements

1. Fees Management Improvements:

   • Introduced BalanceDirection and DiffBalance structs to represent and handle balance movements.
   • Updated the FeesManager trait to include methods for calculating fee differences (calculate_diff_fees) and collecting fees with references (collect_fees).

2. Subscription Management Enhancements:

   • Updated subscription creation, updating, and removal processes to handle fee and deposit differences.
   • Added methods to manage and calculate storage deposits (calculate_diff_deposit).
   • Improved the event system to emit detailed subscription-related events such as FeesCollected and SubscriptionRemoved.

3. Fee Calculation Logic:

   • Implemented tiered fee calculation with predefined discount tiers.
   • Enhanced linear fee calculator to handle fee differences.

4. Testing Enhancements:

   • Added comprehensive tests for fee calculations, fee collection, subscription updates, and edge cases like insufficient balance.
   • Improved mock setup to better simulate real scenarios.

5. Code Refactoring:

   • Refactored existing code for better readability and maintainability.
   • Consolidated common logic and improved error handling.