Payrex-python

- Add billing_details_collection support for checkout sessions.
- Add billing_details_collection support for billing statements.

- Add statement_descriptor support for payment intents.
- Add statement_descriptor support for billing statements.

- Resolve bug.

- Add send billing statement via email endpoint.

- Add billing statement endpoints
- Add customer endpoints

Breaking change
- Standardize the use of arrays in resources. The `payment_intent` attribute of CheckoutSession resource is now an array. Previously, this attribute is a PaymentIntent resource.

- Add amount_capturable and amount_received for hold then capture partial amount support.