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/util/json_parsing.dart

@@ -0,0 +1,8 @@
+String cleanJson(String maybeInvalidJson) {
+  if (maybeInvalidJson.contains('```')) {
+    final withoutLeading = maybeInvalidJson.split('```json').last;
+    final withoutTrailing = withoutLeading.split('```').first;
+    return withoutTrailing;
+  }
+  return maybeInvalidJson;
+}