Add Namespace and ProjectStatistics to GraphQL API [ gitlab-org/gitlab-ce!28998 compliant ]

What does this MR do?

This MR fix gitlab-org/gitlab-ce!28277 in order to be compatible with the ongoing refactoring in gitlab-org/gitlab-ce!28998

1. If gitlab-org/gitlab-ce!28998 is merged before gitlab-org/gitlab-ce!28277 we can merge this instead.
2. otherwise https://gitlab.com/gitlab-org/gitlab-ce/commit/4a1dbceca36015fbc6f146e07dae8b7223b15829 can be cherry-picked into gitlab-org/gitlab-ce!28998

/cc @digitalmoksha

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

/label ~Delivery GraphQL backend