Feat: Implement payment functions into ChainEvents contract with tests

[emarc99]

• Closes: Implement Payment Functionality for Paid Events #189

Key implementation details:

1. Payment Validation:

   • Checks that the event exists and is of paid type
   • Verifies the event isn't closed
   • Ensures the user hasn't already paid for this event
   • Ensures that users registered for an event before paying for it

2. Payment Tracking:

   • Stores payment details in paid_events mapping (user → (event_id, amount))
   • Tracks total amount collected per event in paid_events_amount
   • Counts tickets sold in paid_event_ticket_count

3. Withdrawal Security:

   • Only event owner can withdraw
   • Checks there are funds to withdraw
   • Resets balance before transfer to prevent reentrancy
   • Uses openzeppelin ERC20 transfer_from call

4. Events:

   • Added PaymentMade and PaymentWithdrawn events
   • Emitted when payments are made or withdrawn

5. Additional Functions:

• Implemented fetch_user_paid_event() to return user's payment info

[emarc99]

@mubarak23 kindly lemme know if something is amiss, thanks!