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/services/firestore.dart

@@ -0,0 +1,25 @@
+import 'package:cloud_firestore/cloud_firestore.dart';
+
+import '../features/recipes/recipe_model.dart';
+
+const recipePath = '/recipes';
+final firestore = FirebaseFirestore.instance;
+
+class FirestoreService {
+  static Future<Null> saveRecipe(Recipe recipe) async {
+    await firestore
+        .collection(recipePath)
+        .doc(recipe.id)
+        .set(recipe.toFirestore());
+  }
+
+  static Future<Null> deleteRecipe(Recipe recipe) async {
+    await firestore.doc("$recipePath/${recipe.id}").delete();
+  }
+
+  static Future<Null> updateRecipe(Recipe recipe) async {
+    await firestore
+        .doc("$recipePath/${recipe.id}")
+        .update(recipe.toFirestore());
+  }
+}