Make Sideloading easier

Nobody has claimed this yet.

# Proposal: Make Sideloading easier
Sideloading an app should be as easy, if not easier than installing an app through a regular installer. Users should not have to go through installing a certificate in order to Sideload an app, just like they don't when installing a traditional desktop app.
It makes the Microsoft Store the only consumer-friendly place to host packaged apps, but what if the app breaks Store Regulations or is an older version of the app?

With WinUI3 Desktop apps, it seems ever so important to make this happen as a lot of these apps will likely break Store Regulations.

Why can't we have something as simple as a popup saying "This app is not signed and could be dangerous. Do you wish to proceed?", or un-ideally but better than nothing, something like macOS.

## Rationale

* It allows pretty much any user to Sideload apps, not just the few that know how to install a packaged app's certificate.

Start with the proposal and the referenced docs/roadmap.md to understand how sideloading fits the Windows App SDK direction. Define the supported consumer installation flow, including how unsigned or externally packaged apps are handled, and document the security and completion criteria before implementation.

Written by the indexing model from the issue text.