resolveEventType

根据事件的主类型 postType 和二级子类型获取到一个唯一的事件序列化器，如果找不到则得到 null。

• "meta_event" (total: 2)

• "lifecycle" ->RawLifecycleEvent

• "heartbeat" ->RawHeartbeatEvent

• "message" (total: 2)

• "group" ->RawGroupMessageEvent

• "private" ->RawPrivateMessageEvent

• "request" (total: 2)

• "group" ->RawGroupRequestEvent

• "friend" ->RawFriendRequestEvent

• "notice" (total: 9)

• "friend_recall" ->RawFriendRecallEvent

• "notify" ->RawNotifyEvent

• "group_decrease" ->RawGroupDecreaseEvent

• "group_admin" ->RawGroupAdminEvent

• "group_increase" ->RawGroupIncreaseEvent

• "group_recall" ->RawGroupRecallEvent

• "group_upload" ->RawGroupUploadEvent

• "group_ban" ->RawGroupBanEvent

• "friend_add" ->RawFriendAddEvent