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/message_widget.dart

@@ -0,0 +1,59 @@
+// 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:flutter_markdown/flutter_markdown.dart';
+
+class MessageWidget extends StatelessWidget {
+  const MessageWidget({
+    super.key,
+    this.text,
+    this.image,
+    required this.isFromUser,
+  });
+
+  final Image? image;
+  final String? text;
+  final bool isFromUser;
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      mainAxisAlignment:
+          isFromUser ? MainAxisAlignment.end : MainAxisAlignment.start,
+      children: [
+        Flexible(
+          child: Container(
+            constraints: const BoxConstraints(maxWidth: 520),
+            decoration: BoxDecoration(
+              color: isFromUser
+                  ? Theme.of(context).colorScheme.primaryContainer
+                  : Theme.of(context).colorScheme.surfaceVariant,
+              borderRadius: BorderRadius.circular(18),
+            ),
+            padding: const EdgeInsets.symmetric(
+              vertical: 15,
+              horizontal: 20,
+            ),
+            margin: const EdgeInsets.only(bottom: 8),
+            child: Column(children: [
+              if (text case final text?) MarkdownBody(data: text),
+              if (image case final image?) image,
+            ]),
+          ),
+        ),
+      ],
+    );
+  }
+}