feat: update transfer

[Akshola00]

Description

This PR addresses and resolves the critical issues outlined in #71 for the Transfer component. The following enhancements and fixes are included:

Issues Fixed

1. Expiration Handling:

   • Added logic to check transfer expiration and emit the TransferExpired event as appropriate.

2. Agent Assignment:

   • Implemented an assign_agent() function with proper admin access control for assigning or updating the assigned_agent.

3. Balance Validation:

   • Introduced checks for sufficient ERC20 balance and allowance before allowing transfers to proceed.

4. Update Events:

   • Now emits TransferUpdated events for changes to updated_at even if the status does not change.

5. Pagination Efficiency:

   • Improved pagination logic to reduce gas costs from sequential iteration.

6. Error Codes:

   • Introduced more specific error codes (e.g., ZERO_AMOUNT, ZERO_RECIPIENT) to provide clear error handling rather than generic messages.

7. Access Control:

   • Added role-based permissions to restrict sensitive operations to admins.

8. Metadata Field:

   • Added validation and/or usage logic for the transfer metadata field.

Acceptance Criteria

• Expired transfers are handled and signaled with proper events.
• Admins can assign agents securely.
• Transfers check balances and allowances before proceeding.
• Comprehensive and specific error messages are returned.
• All changes are covered by tests.

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Closes #71

[Pvsaint]

Nice job!