Adding dynamic_theme and gemini_tasks (#2287)

Adds two new samples that use the Google Generative AI SDK for Dart.
Both were created by @rodydavis. I'm merely the uploader so to speak.

## 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.

dynamic_theme/lib/widgets/api_key_widget.dart

@@ -0,0 +1,88 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import 'package:flutter/material.dart';
+import 'package:url_launcher/link.dart';
+
+import 'text_field_decoration.dart';
+
+class ApiKeyWidget extends StatelessWidget {
+  ApiKeyWidget({
+    super.key,
+    required this.onSubmitted,
+    required this.title,
+  });
+
+  final String title;
+  final ValueChanged onSubmitted;
+  final _textController = TextEditingController();
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text(title),
+      ),
+      body: Center(
+        child: Padding(
+          padding: const EdgeInsets.all(8.0),
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              const Text(
+                'To use the Gemini API, you\'ll need an API key. '
+                'If you don\'t already have one, '
+                'create a key in Google AI Studio.',
+                textAlign: TextAlign.center,
+              ),
+              const SizedBox(height: 8),
+              Link(
+                uri: Uri.https('aistudio.google.com', '/app/apikey'),
+                target: LinkTarget.blank,
+                builder: (context, followLink) => TextButton(
+                  onPressed: followLink,
+                  child: const Text('Get an API Key'),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+      bottomNavigationBar: BottomAppBar(
+        padding: const EdgeInsets.all(8),
+        child: Row(
+          children: [
+            Expanded(
+              child: TextField(
+                decoration: textFieldDecoration(context, 'Enter your API key'),
+                controller: _textController,
+                obscureText: true,
+                onSubmitted: (value) {
+                  onSubmitted(value);
+                },
+              ),
+            ),
+            const SizedBox(height: 8),
+            TextButton(
+              onPressed: () {
+                onSubmitted(_textController.value.text);
+              },
+              child: const Text('Submit'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}