Feature proposal:Provide BackgroundDownloader in Windows App SDK

Nobody has claimed this yet.

### Describe the bug

Please provide BackgroundDownloader in Windows App SDK

In the Windows UWP namespace, there is an API called BackgroundDownloader that can download files in the background, however, this API can only run in the container, which is limited in desktop applications and difficult to call. I hope the Windows App SDK team will seriously consider redesigning a BackgroundDownloader API for downloading files in the Windows App SDK. This API breaks through the limitations of UWP BackgroundDownloader and can download files smoothly. When downloading files, you can call the newly added delivery optimization interface of Windows 10 1809.

在 Windows UWP 命名空间中，有一个 API 叫 BackgroundDownloader 可以在后台下载文件，然而，这个 API 只能在容器中运行，这在桌面应用中存在限制，很难调用。希望 Windows App SDK 团队能认真考虑一下，重新设计一个在 Windows App SDK 下载文件的 BackgroundDownloader API，该 API 突破 UWP BackgroundDownloader 的限制，可以顺利下载文件，下载文件时，可以调用 Windows 10 1809 新添加的传递优化接口。

UWP BackgroundDownloader API:https://learn.microsoft.com/zh-cn/uwp/api/windows.networking.backgroundtransfer.backgrounddownloader?view=winrt-22621
Desktop app use BackgroundDownloader API problem:https://github.com/microsoft/microsoft-ui-xaml/issues/6952
Delivery optimization API : https://learn.microsoft.com/zh-cn/windows/win32/api/_delivery_optimization/

### Steps to reproduce the bug

See describe

### Expected behavior

Provide Microsoft.Windows.Networking.Backgroundtransfer namespace and BackgroundDownloader class API

### Screenshots

None

### NuGet package version

Windows App SDK 1.5.3: 1.5.240428000

### Packaging type

Packaged (MSIX)

### Windows version

Windows 11 version 22H2 (22621, 2022 Update)

### IDE

Visual Studio 2022

### Additional context

None

Start by comparing the linked UWP BackgroundDownloader API with the linked Delivery Optimization API and the desktop limitations described in microsoft-ui-xaml#6952. No implementation file, entry point, or test is identified; the work would need an agreed Windows App SDK API design and validation that desktop apps can perform background downloads outside the UWP container.

Written by the indexing model from the issue text.