Coupon code input for new SaaS purchase

What does this MR do and why?

Adds a new field on purchase flow to let customers apply a coupon code. This MR is about adding the field only, it will be wired up in a follow-up MR.

Screenshots or screen recordings

How to set up and validate locally

Instructions for validation here.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383203