EDU automation - create API to request the second coupon code

There are frequent cases when a customer applied a coupon to a Self-Managed subscription instead of SaaS and vice versa in the context of GitLab for Education. @daniel-murphy suggested to give a customer ability to request one more if they need without passing a one more verification circle. UI for it already implemented in the https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/13626. We just need to create the endpoint for the button that is already in place and show some success message if request succeed.

In the future this functionality could be also re-used to decrease the amount of verifications if account is already verified not a long time ago (for example once per 2-3 years instead of each year as it works currently).