Use account id as part of the namespace to init Zuora iframe

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR is a step towards solving the scenario of gitlab-org/gitlab#341564 and is the frontend side of it. The work has been split to make it easier to review.

Retrieve the account id and pass it to the FE	!71266 (merged)
Use account id as part of the namespace to init Zuora iframe	👈 you are here

The MR goes on to polish a bit the GraphQL implementation:

• using more consistent names (with the backend)
• assuming the account ID will be linked to a namesapce
• undoing the relation between namespace and subscription

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

There are no UI changes.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Example below:

1. Make sure your CustomersDot is running locally
2. Make sure to have set GITLAB_LICENSE_MODE=test on gdk (see here)
3. Visit /-/subscriptions/buy_minutes/?selected_group=<your-group-id>

You can inspect the Vue component and see the empty accountId is correctly passed.

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.