Add support for async plugins

src/DesktopMagicPluginAPI/AsyncPlugin.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Drawing;
+using System.Threading.Tasks;
+
+namespace DesktopMagic.Api;
+
+/// <summary>
+/// Provides an abstract base class for plugins that execute asynchronously. Derive from this class to implement plugins
+/// whose main logic runs in a non-blocking manner using asynchronous operations.
+/// </summary>
+/// <remarks>AsyncPlugin is intended for scenarios where plugin execution should not block the calling thread.
+/// Override the MainAsync method to implement asynchronous plugin behavior. The synchronous Main method is sealed and
+/// obsolete; it cannot be used for execution and will always throw an exception. Use MainAsync for all plugin logic.
+/// The MainAsync method is typically invoked when the UpdateInterval elapses, allowing periodic asynchronous
+/// execution.</remarks>
+public abstract class AsyncPlugin : Plugin
+{
+    /// <summary>
+    /// Occurs when the <see cref="Plugin.UpdateInterval"/> elapses.
+    /// </summary>
+    /// <returns></returns>
+    public abstract Task<Bitmap?> MainAsync();
+
+    /// <summary>
+    /// This method should not be called directly! Override and use <see cref="MainAsync"/> for asynchronous plugin operations.
+    /// </summary>
+    /// <remarks>This method only exists to fulfill base class requirements.</remarks>
+    /// <returns>Calling it will always result in an <see cref="InvalidOperationException"/>.</returns>
+    /// <exception cref="InvalidOperationException">Thrown if this method is called directly. Use MainAsync for plugin execution instead.</exception>
+    public sealed override Bitmap? Main()
+    {
+        throw new InvalidOperationException($"AsyncPlugin.Main() should not be called directly. Override {nameof(MainAsync)} instead.");
+    }
+}