Generate graphql schema during gitlab-update

See: gitlab!53974 (comment 507856882)

This MR has GDK generate the schema files during gdk update.

This is a backwards compatible change, since generating this file now is fine - it may result in a dirty status (until gitlab!53974 (merged) is merged and the generated file gets ignored), but this is no different to generating the DB schema.

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.