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/recipe_model.dart

@@ -0,0 +1,110 @@
+import 'dart:convert';
+
+import 'package:ai_recipe_generation/util/json_parsing.dart';
+import 'package:google_generative_ai/google_generative_ai.dart';
+
+class Recipe {
+  Recipe({
+    required this.title,
+    required this.id,
+    required this.description,
+    required this.ingredients,
+    required this.instructions,
+    required this.cuisine,
+    required this.allergens,
+    required this.servings,
+    required this.nutritionInformation,
+    this.rating = -1,
+  });
+
+  final String id;
+  final String title;
+  final String description;
+  final List<String> ingredients;
+  final List<String> instructions;
+  final String cuisine;
+  final List<String> allergens;
+  final String servings;
+  final Map<String, dynamic> nutritionInformation;
+  int rating;
+
+  factory Recipe.fromGeneratedContent(GenerateContentResponse content) {
+    /// failures should be handled when the response is received
+    assert(content.text != null);
+
+    final validJson = cleanJson(content.text!);
+    final json = jsonDecode(validJson);
+
+    if (json
+        case {
+          "ingredients": List<dynamic> ingredients,
+          "instructions": List<dynamic> instructions,
+          "title": String title,
+          "id": String id,
+          "cuisine": String cuisine,
+          "description": String description,
+          "servings": String servings,
+          "nutritionInformation": Map<String, dynamic> nutritionInformation,
+          "allergens": List<dynamic> allergens,
+        }) {
+      return Recipe(
+          id: id,
+          title: title,
+          ingredients: ingredients.map((i) => i.toString()).toList(),
+          instructions: instructions.map((i) => i.toString()).toList(),
+          nutritionInformation: nutritionInformation,
+          allergens: allergens.map((i) => i.toString()).toList(),
+          cuisine: cuisine,
+          servings: servings,
+          description: description);
+    }
+
+    throw JsonUnsupportedObjectError(json);
+  }
+
+  Map<String, Object?> toFirestore() {
+    return {
+      'id': id,
+      'title': title,
+      'instructions': instructions,
+      'ingredients': ingredients,
+      'cuisine': cuisine,
+      'rating': rating,
+      'allergens': allergens,
+      'nutritionInformation': nutritionInformation,
+      'servings': servings,
+      'description': description,
+    };
+  }
+
+  factory Recipe.fromFirestore(Map<String, Object?> data) {
+    if (data
+        case {
+          "ingredients": List<dynamic> ingredients,
+          "instructions": List<dynamic> instructions,
+          "title": String title,
+          "id": String id,
+          "cuisine": String cuisine,
+          "description": String description,
+          "servings": String servings,
+          "nutritionInformation": Map<String, dynamic> nutritionInformation,
+          "allergens": List<dynamic> allergens,
+          "rating": int rating
+        }) {
+      return Recipe(
+        id: id,
+        title: title,
+        ingredients: ingredients.map((i) => i.toString()).toList(),
+        instructions: instructions.map((i) => i.toString()).toList(),
+        nutritionInformation: nutritionInformation,
+        allergens: allergens.map((i) => i.toString()).toList(),
+        cuisine: cuisine,
+        servings: servings,
+        description: description,
+        rating: rating,
+      );
+    }
+
+    throw "Malformed Firestore data";
+  }
+}