feat: added payment gateway system

[adityacosmos24]

Payment Gateway System

Overview

This pull request introduces the implementation for the Payment Gateway System . The solution demonstrates the application of design principles to enable extensibility and abstraction for integrating multiple payment gateways in a software system.

Key Features

• Abstract Payment Gateway Interface:
  Defines a standard contract for all payment gateway integrations (e.g., process payment, refund, etc.).
• Concrete Payment Gateway Implementations:
  Adds support for at least two real or sample gateways (e.g., PayPal, Stripe, Razorpay, etc.), each implementing the abstract interface.
• Client/Service Layer Integration:
  Demonstrates usage of these gateways in a way such that switching between gateways is easy and requires minimal code changes.
• Demonstration/Test Classes:
  Includes main/test classes with sample transactions to show correct integration and OCP compliance (Open/Closed Principle).

Design Principles Illustrated

• Abstraction to generalize payment operations.
• Open/Closed Principle—new gateways can be added without modifying existing code.
• Separation of Concerns between payment logic and high-level application logic.

Review & Testing

• Code compiles and runs as expected
• Demonstration code shows successful payment flows for multiple gateways
• Easy to plug in new payment gateways by implementing the interface

---

Closses #37