feat: modify GroupAtInfoItem private to public (#186)

Author: Antenbabby

src/main/java/io/github/doocs/im/model/callback/AfterSendGroupMsgCallback.java

@@ -100,7 +100,7 @@ public class AfterSendGroupMsgCallback implements Serializable {
     private List<GroupAtInfoItem> groupAtInfo;
 
 
-    private static class GroupAtInfoItem {
+    public static class GroupAtInfoItem {
         /**
          * 群组@全体成员标识，0-非全体@，1-全体@
          */