feat: add JavaScript scripting foundation

Introduces a JavaScript setup script that defines the `ad` helper object.
This `ad` object exposes APIs for manipulating request data, accessing response details, and managing environment variables within scripts.

Adds an `evaluate` function to execute JS code using the `flutter_js` runtime and log results or errors.
This establishes the core infrastructure for pre-request and post-response scripting.

lib/services/flutter_js_service.dart

@@ -1,3 +1,6 @@
+import 'dart:developer';
+
+import 'package:flutter/services.dart';
 import 'package:flutter_js/flutter_js.dart';
 
 late JavascriptRuntime jsRuntime;
@@ -9,3 +12,12 @@ void initializeJsRuntime() {
 void disposeJsRuntime() {
   jsRuntime.dispose();
 }
+
+void evaluate(String code) {
+  try {
+    JsEvalResult jsResult = jsRuntime.evaluate(code);
+    log(jsResult.stringResult);
+  } on PlatformException catch (e) {
+    log('ERROR: ${e.details}');
+  }
+}