models/payment_request library

1. unified_payment package
2. documentation
3. models/payment_request.dart

unified_payment package

1. Libraries
2. models/payment_config
3. models/payment_request
4. models/payment_response
5. payment_service
6. providers/base_provider
7. providers/paypal_provider
8. providers/paystack_provider
9. providers/razorpay_provider
10. providers/stripe_provider
11. unified_payment
12. widgets/payment_webview