Send idempotency key when purchasing new subscription

What does this MR do and why?

Sends idempotency key to the API when purchasing new subscription for a group

Related to https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/7448

Screenshots or screen recordings

Scenario	Screenshot
Idempotency key regeneration	Screen recording Screen_Recording_2023-10-09_at_12.06.30_PM
Successful purchase	Screen recording Screen_Recording_2023-10-05_at_10.45.44_AM
Successful purchase after using a card with insufficient funds	Screen recording Screen_Recording_2023-10-05_at_11.03.36_AM
Unprocessable Content - 422	Screen recording Screen_Recording_2023-10-05_at_10.34.57_AM
Internal Server Error - 500	Screen recording Screen_Recording_2023-10-05_at_10.58.09_AM

How to set up and validate locally

Application setup

1. Customers Dot needs to be setup in order to view the billing page for a group
2. Start GDK in SaaS mode GITLAB_SIMULATE_SAAS=1 gdk start

Subscription purchase

1. Create a Group
2. Go to Settings > Billing
3. Click on Upgrade CTA

Error scenarios

1. Unprocessable content - 422
   • Update create method here to be:

     def create
       respond_422
     end

2. Internal server error - 500
   • Update create method here to be:

     def create
       render status: :service_unavailable
     end

3. Card with insufficient funds - 4000000000009995. More cards which will result in declined payment can be found here - https://stripe.com/docs/testing#declined-payments

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.