Use PackageHelper for graphql download

What does this merge request do and why?

This MR uses the PackageHelper to download the GraphQL schema files. See gitlab!165534 (merged) for the implementation in gitlab-org/gitlab.

How to set up and validate locally

1. Run rm gitlab/tmp/tests/graphql/*
2. Run GDK_SELF_UPDATE=0 gdk update
3. See that gitlab/tmp/tests/graphql/graphql_schema.{json,graphql} were generated correctly

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.