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/macos/Flutter/Flutter-Debug.xcconfig

@@ -0,0 +1,2 @@
+#include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.debug.xcconfig"
+#include "ephemeral/Flutter-Generated.xcconfig"

ai_recipe_generation/macos/Flutter/Flutter-Release.xcconfig

@@ -0,0 +1,2 @@
+#include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.release.xcconfig"
+#include "ephemeral/Flutter-Generated.xcconfig"

ai_recipe_generation/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -0,0 +1,22 @@
+//
+//  Generated file. Do not edit.
+//
+
+import FlutterMacOS
+import Foundation
+
+import cloud_firestore
+import device_info_plus
+import file_selector_macos
+import firebase_core
+import path_provider_foundation
+import shared_preferences_foundation
+
+func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
+  FLTFirebaseFirestorePlugin.register(with: registry.registrar(forPlugin: "FLTFirebaseFirestorePlugin"))
+  DeviceInfoPlusMacosPlugin.register(with: registry.registrar(forPlugin: "DeviceInfoPlusMacosPlugin"))
+  FileSelectorPlugin.register(with: registry.registrar(forPlugin: "FileSelectorPlugin"))
+  FLTFirebaseCorePlugin.register(with: registry.registrar(forPlugin: "FLTFirebaseCorePlugin"))
+  PathProviderPlugin.register(with: registry.registrar(forPlugin: "PathProviderPlugin"))
+  SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
+}