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/windows/flutter/generated_plugin_registrant.cc

@@ -0,0 +1,20 @@
+//
+//  Generated file. Do not edit.
+//
+
+// clang-format off
+
+#include "generated_plugin_registrant.h"
+
+#include <cloud_firestore/cloud_firestore_plugin_c_api.h>
+#include <file_selector_windows/file_selector_windows.h>
+#include <firebase_core/firebase_core_plugin_c_api.h>
+
+void RegisterPlugins(flutter::PluginRegistry* registry) {
+  CloudFirestorePluginCApiRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("CloudFirestorePluginCApi"));
+  FileSelectorWindowsRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("FileSelectorWindows"));
+  FirebaseCorePluginCApiRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("FirebaseCorePluginCApi"));
+}