Support additional payment methods for e-commerce orders (Invoice, PayPal, Google Pay, Apple Pay)

🔍 Problem Statement

E-commerce SKU orders only support credit card and BACS, limiting checkout options for buyers who prefer other payment methods.

💡 User Story

As a client admin, I want to offer invoice-based payment for e-commerce orders, so that buyers can pay via credit card or offline payment against an invoice.

As a client admin, I want to accept PayPal, Google Pay, and Apple Pay for e-commerce orders, so that buyers have more checkout options.

As a buyer on the default checkout UI, I want to select from these payment methods, so that I can pay however I prefer.

🎯 Definition of Done (DoD)

✔ Given e-commerce checkout, when a customer selects "Send Invoice," then an invoice is generated and payable via credit card or offline payment.

✔ Given e-commerce checkout, when a customer selects PayPal, Google Pay, or Apple Pay, then the order completes and is paid through that method.

✔ This change will impact UI and API, specifically the e-commerce checkout endpoint(s) and the default checkout UI's payment method selector.

✔ This solution will include the following limitations: requires each payment gateway (PayPal, Google Pay, Apple Pay) to be configured and enabled at the platform level.