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.

---------

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>

routers/web/repo/issue.go

@@ -458,7 +458,7 @@ func UpdateIssueAssignee(ctx *context.Context) {
 				return
 			}
 
-			valid, err := access_model.CanBeAssigned(ctx, assignee, issue.Repo, issue.IsPull)
+			valid, err := access_model.CanBeAssigned(ctx, assignee, issue.Repo)
 			if err != nil {
 				ctx.ServerError("canBeAssigned", err)
 				return