Create shift4-sofort-support

[sujith3006]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

This PR introduces Sofort payment method support for the Shift4 provider.
It adds a new PaymentMethodType::Sofort, implements request handling in the Shift4 client, and enables European users to initiate payments using Sofort via Shift4 APIs.

Key changes include:

• Added Sofort variant to payment method enums
• Implemented Sofort payment flow in Shift4Client
• Updated request models to accept Sofort method
• Added basic unit test to validate Sofort payment handling

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

(No breaking changes were introduced. Existing card/payment flows remain unaffected.)

Motivation and Context

This feature enables Sofort, a widely-used European bank transfer method, within the Shift4 integration.
It expands platform support for EU merchants and improves localization of payment options.
Requested as part of enhancing alternative payment methods support.

How did you test it?

• ✅ Added unit test for PaymentMethodType::Sofort
• ✅ Verified request body structure for Shift4 Sofort flow
• 🔍 Manual validation with mocked Shift4 API response

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible

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