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/.metadata

@@ -0,0 +1,45 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: "2e9cb0aa71a386a91f73f7088d115c0d96654829"
+  channel: "stable"
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+      base_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+    - platform: android
+      create_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+      base_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+    - platform: ios
+      create_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+      base_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+    - platform: linux
+      create_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+      base_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+    - platform: macos
+      create_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+      base_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+    - platform: web
+      create_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+      base_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+    - platform: windows
+      create_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+      base_revision: 2e9cb0aa71a386a91f73f7088d115c0d96654829
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'