Adds ai_recipe_generation sample (#2242)

Adding the demo app from my I/O talk. Because AI.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

ai_recipe_generation/lib/features/recipes/recipes_view_model.dart

@@ -0,0 +1,31 @@
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:flutter/cupertino.dart';
+
+import '../../services/firestore.dart';
+import 'recipe_model.dart';
+
+class SavedRecipesViewModel extends ChangeNotifier {
+  List<Recipe> recipes = [];
+
+  final recipePath = '/recipes';
+  final firestore = FirebaseFirestore.instance;
+
+  SavedRecipesViewModel() {
+    firestore.collection(recipePath).snapshots().listen((querySnapshot) {
+      recipes = querySnapshot.docs.map((doc) {
+        final data = doc.data();
+        return Recipe.fromFirestore(data);
+      }).toList();
+      notifyListeners();
+    });
+  }
+
+  void deleteRecipe(Recipe recipe) {
+    FirestoreService.deleteRecipe(recipe);
+  }
+
+  void updateRecipe(Recipe recipe) {
+    FirestoreService.updateRecipe(recipe);
+    notifyListeners();
+  }
+}