feat: add support for pre-request scripts

Introduces `preRequestScript` and `postRequestScript` fields to the request model to store user-defined scripts.

Implements a service using `flutter_js` to execute JavaScript pre-request scripts before a request is sent.
The script can access and modify request data (like headers, body, URL) and environment variables.

Adds bridging to forward JavaScript `console.log`, `console.warn`, and `console.error` calls to the Dart console for easier debugging

lib/models/request_model.dart

@@ -22,6 +22,8 @@ class RequestModel with _$RequestModel {
     HttpResponseModel? httpResponseModel,
     @JsonKey(includeToJson: false) @Default(false) bool isWorking,
     @JsonKey(includeToJson: false) DateTime? sendingTime,
+    @Default("") String preRequestScript,
+    @Default("") String postRequestScript,
   }) = _RequestModel;
 
   factory RequestModel.fromJson(Map<String, Object?> json) =>