chore: add `dns` type for kcp protocol

see https://xtls.github.io/config/transports/mkcp.html#headerobject

web/html/xui/form/stream/stream_kcp.html

@@ -8,6 +8,7 @@
             <a-select-option value="wechat-video">WeChat</a-select-option>
             <a-select-option value="dtls">DTLS 1.2</a-select-option>
             <a-select-option value="wireguard">WireGuard</a-select-option>
+            <a-select-option value="dns">DNS</a-select-option>
         </a-select>
     </a-form-item>
     <a-form-item>