[RIQ-6320] added back "getUserAgent" method, which was removed in a merge

Author: greg-aofl

lib/src/base.dart

@@ -224,6 +224,12 @@ class FlutterWebviewPlugin {
     return res;
   }
 
+  ///Get userAgent
+  Future<String> getUserAgent() async {
+    final String userAgent = await _channel.invokeMethod('getUserAgent');
+    return userAgent;
+  }
+
   /// Close the Webview
   /// Will trigger the [onDestroy] event
   Future<void> close() async {