feat: add new Toast.

src/Ursa/Controls/Toast/IManagedToastManager.cs

@@ -0,0 +1,22 @@
+using Avalonia.Metadata;
+
+namespace Ursa.Controls;
+
+/// <summary>
+/// Represents a toast manager that can show arbitrary content.
+/// Managed toast managers can show any content.
+/// </summary>
+/// <remarks>
+/// Because toast managers of this type are implemented purely in managed code, they
+/// can display arbitrary content, as opposed to toast managers which display toasts
+/// using the host operating system's toast mechanism.
+/// </remarks>
+[NotClientImplementable]
+public interface IManagedToastManager : IToastManager
+{
+    /// <summary>
+    /// Shows a toast.
+    /// </summary>
+    /// <param name="content">The content to be displayed.</param>
+    void Show(object content);
+}