Add Promotion code parameter to `SetupPaymentSheetParameters`

[Tom3652]

Is your feature request related to a problem? (please describe) I would like to be able to add a promotion code to a subscription. Currently there is no way to pass the promotion code to Stripe.instance.initPaymentSheet.

Describe the solution you'd like I would like to have a new parameter in SetupPaymentSheetParameters that is called promotionCode that displays the equivalent of this field from the Stripe documentation :

Describe alternatives you've considered Designing a custom UI to enter a promo code before showing Stripe modal and use it inside the backend as a separate request to check if the promotion exists.

Additional context Thanks for the package !