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
Edited by Walmyr Lima e Silva Filho