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/README.md

@@ -0,0 +1,15 @@
+This is a demo app created for the Google I/O talk "Gemini API and Flutter: Practical, AI-driven apps with Google AI tools"
+
+## Running the app 
+
+Before running the app:
+* Get an API key from [ai.google.dev](ai.google.dev).
+* Create a Firebase project, and install the Flutter Firebase CLI, and configure the platforms that you want to run this app on. You can find instructions by following [steps 2-4 of the Get to know Firebase for Flutter](https://firebase.google.com/codelabs/firebase-get-to-know-flutter?hl=en#2) codelab.
+
+Then, pass the API key in with dart define when running the app:
+
+```bash
+flutter run --dart-define=API_KEY=your_api_key
+```
+
+