Add Option to download package manifests along with the installers

[Gabriel Dufresne (GabrielDuf)]

Adds an opt-in setting so that, when downloading a WinGet package's installer, UniGetUI also saves the package's YAML manifest next to it — matching what native winget download produces. This gives users maintaining offline/reproducible software repositories a self-contained snapshot (installer + metadata) instead of just the bare .exe/.msi. #4817

How it works

• New setting WinGetDownloadFullManifest (off by default), exposed as a checkbox in the WinGet section of the package-manager settings page (both the WinUI and Avalonia settings UIs).
• New WinGetManifestDownloadOperation that invokes pinget/winget download --download-directory … instead of the plain HTTP installer grab. The download verb writes the installer plus a flattened YAML manifest.
• The download dispatch in AppOperationHelper (WinUI) and AvaloniaPackageOperationHelper (Avalonia) routes WinGet packages through the new operation when the setting is enabled, and falls back to the existing DownloadOperation for every other manager / when the toggle is off. Because the verb writes a folder rather than a single file, the single-package path uses a folder picker when the setting is on.