Add bottom to CupertinoNavigationBar to allow for a double-row nav bar (#155959)

Fixes [Support segmented controls in nav bars and double row nav bars](https://github.com/flutter/flutter/issues/10469)

<img width="270" height="600"  alt="Screenshot 2024-09-30 at 3 09 04 PM" src="https://github.com/user-attachments/assets/4003116f-69dd-4f8f-a185-6ca151b74d2d">

examples/api/lib/cupertino/nav_bar/cupertino_navigation_bar.1.dart

@@ -0,0 +1,72 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+
+/// Flutter code sample for [CupertinoNavigationBar] showing a
+/// [CupertinoSearchTextField] with padding at the bottom of the navigation bar.
+
+void main() => runApp(const NavBarApp());
+
+class NavBarApp extends StatelessWidget {
+  const NavBarApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const CupertinoApp(
+      theme: CupertinoThemeData(brightness: Brightness.light),
+      home: NavBarExample(),
+    );
+  }
+}
+
+class NavBarExample extends StatefulWidget {
+  const NavBarExample({super.key});
+
+  @override
+  State<NavBarExample> createState() => _NavBarExampleState();
+}
+
+class _NavBarExampleState extends State<NavBarExample> {
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: const CupertinoNavigationBar(
+        middle: Text('CupertinoNavigationBar Sample'),
+        bottom: _NavigationBarSearchField(),
+        automaticBackgroundVisibility: false,
+      ),
+      child: Column(
+        children: <Widget>[
+          Container(height: 50, color: CupertinoColors.systemRed),
+          Container(height: 50, color: CupertinoColors.systemGreen),
+          Container(height: 50, color: CupertinoColors.systemBlue),
+          Container(height: 50, color: CupertinoColors.systemYellow),
+        ],
+      ),
+    );
+  }
+}
+
+
+class _NavigationBarSearchField extends StatelessWidget implements PreferredSizeWidget {
+  const _NavigationBarSearchField();
+
+  static const double padding = 8.0;
+  static const double searchFieldHeight = 35.0;
+
+  @override
+  Widget build(BuildContext context) {
+    return const Padding(
+      padding: EdgeInsets.symmetric(horizontal: padding, vertical: padding),
+      child: SizedBox(
+        height: searchFieldHeight,
+        child: CupertinoSearchTextField()
+      ),
+    );
+  }
+
+  @override
+  Size get preferredSize => const Size.fromHeight(searchFieldHeight + padding * 2);
+}

examples/api/test/cupertino/nav_bar/cupertino_navigation_bar.1_test.dart

@@ -0,0 +1,27 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+import 'package:flutter_api_samples/cupertino/nav_bar/cupertino_navigation_bar.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('CupertinoNavigationBar with bottom widget', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.NavBarApp(),
+    );
+
+    final Finder navBarFinder = find.byType(CupertinoNavigationBar);
+    final Finder searchFieldFinder = find.byType(CupertinoSearchTextField);
+
+    expect(navBarFinder, findsOneWidget);
+    expect(searchFieldFinder, findsOneWidget);
+
+    // The bottom widget is bounded by the navigation bar.
+    expect(
+      tester.getBottomLeft(searchFieldFinder).dy,
+      lessThan(tester.getBottomLeft(navBarFinder).dy),
+    );
+  });
+}