[MM-55323] Allow end users to fetch the group members list of groups which allow @-mentions (#26551)

* Allow end users to fetch the group members list of groups which allow @-mentions

* Update server/channels/api4/group_test.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Fix test name

* Move into subtest

---------

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

server/public/model/group_member.go

@@ -21,3 +21,8 @@ func (gm *GroupMember) IsValid() *AppError {
 	}
 	return nil
 }
+
+type GroupMemberList struct {
+	Members []*User `json:"members"`
+	Count   int     `json:"total_member_count"`
+}