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/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -0,0 +1,12 @@
+//
+//  Generated file. Do not edit.
+//
+
+import FlutterMacOS
+import Foundation
+
+import url_launcher_macos
+
+func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
+  UrlLauncherPlugin.register(with: registry.registrar(forPlugin: "UrlLauncherPlugin"))
+}