Update issue resource to allow assignees on issue fabrication via API

What does this MR do?

I was making this change on !16963 (closed), but decided to break it down into smaller pieces in favor of iteration.

Related issues: gitlab-org/quality/testcases#176 (closed), and gitlab-org/quality/testcases#235 (closed).

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