feat(work): 新增管理员变更相关的回调通知事件模型

src/SKIT.FlurlHttpClient.Wechat.Work/Events/Service/ChangeAppAdminEvent.cs

@@ -0,0 +1,15 @@
+namespace SKIT.FlurlHttpClient.Wechat.Work.Events
+{
+    /// <summary>
+    /// <para>表示 EVENT.change_app_admin 事件的数据。</para>
+    /// <para>REF: https://open.work.weixin.qq.com/api/doc/90001/90143/95038 </para>
+    /// </summary>
+    public class ChangeAppAdminEvent : WechatWorkEvent, WechatWorkEvent.Types.IXmlSerializable
+    {
+        /// <summary>
+        /// 获取或设置应用 ID。
+        /// </summary>
+        [System.Xml.Serialization.XmlElement("AgentID")]
+        public int AgentId { get; set; }
+    }
+}

test/SKIT.FlurlHttpClient.Wechat.Work.UnitTests/EventSamples/Service/ChangeAppAdminEvent.xml

@@ -0,0 +1,8 @@
+<xml>
+    <ToUserName><![CDATA[toUser]]></ToUserName>
+    <FromUserName><![CDATA[sys]]></FromUserName>
+    <CreateTime>1348831860</CreateTime>
+    <MsgType><![CDATA[event]]></MsgType>
+    <Event><![CDATA[change_app_admin]]></Event>
+    <AgentID>1</AgentID>
+</xml>