Expose comments for designs in GraphQL

What does this MR do

This includes the changes from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29212 and then uses those to expose comments for Designs

This is part of https://gitlab.com/gitlab-org/gitlab-ee/issues/9489 and https://gitlab.com/gitlab-org/gitlab-ce/issues/62825

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