🛠️ Feature: Mercado Pago Integration

Description:
Added the necessary files to integrate Mercado Pago, a widely recognized payment gateway in Latin America, developed and maintained by the e-commerce giant Mercado Libre. The payment method supports webhooks and follows the same structure as other payment methods in the system.

Key Changes:

• Included Mercado Pago SDK and configured authentication with access tokens.
• Implemented payment creation and redirection to Mercado Pago checkout.
• Handled webhook callbacks to update order/payment status.