MLTrainer: setup .msix builds

[jonathanpeppers]

The current Release app using WindowsPackageType=None is broken -- file pickers crash the app.

We can probably use a .msix, we will need to create a self-signed certificate and commit it to source control.

Examples/links:

• https://learn.microsoft.com/dotnet/maui/windows/deployment/overview
• https://github.com/jfversluis/MauiGithubActionsSample/blob/main/.github/workflows/dotnet-windows.yml