add sdk UA

Author: hhxsv5

src/Api.php

@@ -13,6 +13,11 @@
 
 abstract class Api
 {
+    /**
+     * @var string SDK Version
+     */
+    const VERSION = '1.1.9';
+
     /**
      * @var string
      */
@@ -192,6 +197,7 @@ public function call($method, $uri, array $params = [], array $headers = [], $ti
             );
             $headers = array_merge($headers, $authHeaders);
         }
+        $headers['User-Agent'] = 'KuCoin-PHP-SDK/' . static::VERSION;
         $request->setHeaders($headers);
 
         $requestId = uniqid();