Add more relations to Group Export

What does this MR do?

This MR adds more relations to group_import_export.yml so that when Group Export is performed there is more information on epics and boards. Epics now should include comments. Boards should include assignees and labels. Both of these changes are EE only as nether epics not board assignees are available in CE.

This is a ~backstage change since Group Import is not available yet and Group Export is not yet released (subject to change). It's API endpoint is not yet used / documented either.

Screenshots

N/A

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A