feat(api): Add assignees APIs (#37330)

Follow
https://docs.github.com/en/enterprise-server@3.20/rest/issues/assignees?apiVersion=2022-11-28

Fix #33576 

And it also fixed some possible dead-lock problem.

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Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

modules/structs/issue.go

@@ -125,6 +125,11 @@ type EditIssueOption struct {
 	ContentVersion *int `json:"content_version"`
 }
 
+// IssueAssigneesOption options for adding/removing issue assignees
+type IssueAssigneesOption struct {
+	Assignees []string `json:"assignees"`
+}
+
 // EditDeadlineOption options for creating a deadline
 type EditDeadlineOption struct {
 	// required:true