Refactor genai package to new modular interface

Reorganized the genai package by removing legacy LLM-related files and introducing a new modular interface under the 'interface' directory. Added provider-specific model classes, centralized constants, and updated the example to use the new API and data structures. Updated exports in genai.dart and improved dependency management.

packages/genai/lib/models/model_provider.dart

@@ -0,0 +1,18 @@
+import 'package:better_networking/better_networking.dart';
+import '../models/models.dart';
+
+abstract class ModelProvider {
+  ModelRequestData get defaultRequestData => throw UnimplementedError();
+
+  HttpRequestModel? createRequest(ModelRequestData? requestData) {
+    throw UnimplementedError();
+  }
+
+  String? outputFormatter(Map x) {
+    throw UnimplementedError();
+  }
+
+  String? streamOutputFormatter(Map x) {
+    throw UnimplementedError();
+  }
+}