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/add_image_widget.dart

@@ -0,0 +1,85 @@
+import 'package:flutter/material.dart';
+import 'package:material_symbols_icons/symbols.dart';
+
+import '../theme.dart';
+
+class AddImage extends StatefulWidget {
+  const AddImage({
+    super.key,
+    required this.onTap,
+    this.height = 100,
+    this.width = 100,
+  });
+
+  final VoidCallback onTap;
+  final double height;
+  final double width;
+
+  @override
+  State<AddImage> createState() => _AddImageState();
+}
+
+class _AddImageState extends State<AddImage> {
+  bool hovered = false;
+  bool tappedDown = false;
+
+  Color get buttonColor {
+    var state = (hovered, tappedDown);
+    return switch (state) {
+      // tapped down state
+      (_, true) => MarketplaceTheme.secondary.withOpacity(.7),
+      // hovered
+      (true, _) => MarketplaceTheme.secondary.withOpacity(.3),
+      // base color
+      (_, _) => MarketplaceTheme.secondary.withOpacity(.3),
+    };
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MouseRegion(
+      onEnter: (event) {
+        setState(() {
+          hovered = true;
+        });
+      },
+      onExit: (event) {
+        setState(() {
+          hovered = false;
+        });
+      },
+      child: GestureDetector(
+        onTapDown: (details) {
+          setState(() {
+            tappedDown = true;
+          });
+        },
+        onTapUp: (details) {
+          setState(() {
+            tappedDown = false;
+          });
+          widget.onTap();
+        },
+        child: SizedBox(
+          width: widget.width,
+          height: widget.height,
+          child: ClipRRect(
+            borderRadius:
+                BorderRadius.circular(MarketplaceTheme.defaultBorderRadius),
+            child: Container(
+              decoration: BoxDecoration(
+                color: buttonColor,
+              ),
+              child: const Center(
+                child: Icon(
+                  Symbols.add_photo_alternate_rounded,
+                  size: 32,
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}