Payment method, billing preference, and invoice APIs

Creem currently exposes transaction search APIs, which are useful for basic billing history. What’s still needed are direct APIs for payment methods, customer billing preferences, and invoices/receipts.

Requested APIs:

• list payment methods and show the default method

• create setup or portal sessions for adding/updating payment methods

• safely update or remove payment methods where supported

• read/update billing email, name, company details, address, tax IDs, invoice metadata, and locale

• expose hosted receipt URLs, invoice PDFs when available, invoice IDs, and receipt/invoice metadata linked to transactions

This would enable apps to build embedded billing history, invoice download, payment method, and billing preference UIs without sending users to the customer portal for every billing-management action.

Reference: There is another feature request that also requests the invoice API, but also White-Label branding for invoices, which is not possible because Creem is an MoR. That's why I decided to create a fresh consolidated request.