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
