Add Avalonia 11 support with generic host integration

[Copilot]

Adds Avalonia 11 UI framework support following the established WPF/WinForms pattern. Enables cross-platform desktop applications with full generic host integration (DI, logging, lifecycle).

Core Library (Dapplo.Microsoft.Extensions.Hosting.Avalonia)

• Interfaces: IAvaloniaBuilder, IAvaloniaContext, IAvaloniaService, IAvaloniaShell
• Implementation: AvaloniaHostedService, AvaloniaThread (STA thread management), internal builders/contexts
• Extensions: ConfigureAvalonia(), UseAvaloniaLifetime() for both IHostBuilder and IHostApplicationBuilder
• Targets: net472, net8.0, net9.0

Usage

var host = new HostBuilder()
    .ConfigureAvalonia(avaloniaBuilder =>
    {
        avaloniaBuilder.UseApplication<App>();
        avaloniaBuilder.UseWindow<MainWindow>();
        
        // Optional AppBuilder configuration
        avaloniaBuilder.ConfigureAppBuilder(appBuilder =>
        {
            appBuilder.UsePlatformDetect().LogToTrace();
        });
    })
    .UseAvaloniaLifetime()
    .Build();

await host.RunAsync();

Windows implement IAvaloniaShell and receive DI-injected dependencies via constructor. Application lifetime tied to host lifetime via ClassicDesktopStyleApplicationLifetime.

Additional Components

• Sample: Dapplo.Hosting.Sample.AvaloniaDemo - working demo with MainWindow, App, and ViewLocator
• Template: dotnet new avaloniahost - project scaffolding template with nuspec
• Docs: README updated with usage examples and template installation

Supports multiple windows, custom applications, and optional ViewLocator for MVVM. All dependencies scanned - no vulnerabilities.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• av-build-tel-api-v1.avaloniaui.net
  • Triggering command: /usr/share/dotnet/dotnet dotnet exec --runtimeconfig /home/REDACTED/.nuget/packages/avalonia.buildservices/0.0.29/tools/netstandard2.0/runtimeconfig.json /home/REDACTED/.nuget/packages/avalonia.buildservices/0.0.29/tools/netstandard2.0/Avalonia.BuildServices.Collector.dll (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Try to add Avalonia</issue_title>
<issue_description>I would like to have a similar experience as there already is for WinUI, WPF and WinForms with Avalonia 11.
This was also discussed on: AvaloniaUI/Avalonia#5241

So create Dapplo.Microsoft.Extensions.Hosting.Avalonia
For this use the Avalonia AppBuilder and allow the application using this framework to configure it's own.
Consider StartWithClassicDesktopLifetime, provide an optional default viewlocator.

Make sure there is a sample project, called Dapplo.Hosting.Sample.AvaloniaDemo and provide a template called Dapplo.Microsoft.Extensions.Hosting.Avalonia.Template with nuspec to generate a project with dotnet new.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Try to add Avalonia #33

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