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/widgets/appbar_shape_border.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart';
+
+class AppBarShapeBorder extends ShapeBorder {
+  final double radius;
+
+  const AppBarShapeBorder(this.radius);
+
+  @override
+  EdgeInsetsGeometry get dimensions => EdgeInsets.zero;
+
+  @override
+  Path getInnerPath(Rect rect, {TextDirection? textDirection}) {
+    return Path(); // Define inner path if needed
+  }
+
+  @override
+  Path getOuterPath(Rect rect, {TextDirection? textDirection}) {
+    // Define your custom shape path here
+    Path path = Path();
+    path.moveTo(rect.left, rect.top);
+    path.lineTo(rect.left, rect.bottom - (radius * 2));
+
+    path.quadraticBezierTo(
+      rect.left,
+      rect.bottom - radius,
+      rect.left + radius,
+      rect.bottom - radius,
+    );
+
+    path.lineTo(rect.right - radius, rect.bottom - radius);
+
+    path.quadraticBezierTo(
+        rect.right, rect.bottom - radius, rect.right, rect.bottom);
+
+    path.lineTo(rect.right, rect.top);
+    path.close();
+    return path;
+  }
+
+  @override
+  void paint(Canvas canvas, Rect rect, {TextDirection? textDirection}) {
+    // Define your painting logic here
+    Paint paint = Paint()..color = Colors.transparent;
+    canvas.drawPath(getOuterPath(rect), paint);
+  }
+
+  @override
+  ShapeBorder scale(double t) {
+    // Implement scaling if needed
+    return this;
+  }
+}