Self signed packaged apps - prompt users to trust certificate

# Proposal: Self signed packaged apps - prompt users to trust certificate
Today it isn't straightforward to install self signed packaged apps. This has caused issues both to developers and users. Not everyone can afford code signing or the Store.

## Summary
When installing through App Installer, ask users if they want to trust the certificate (app developer) that's embedded in the package. This will make installing self-signed packages easier.

## Rationale

* Make it easier for users and developers
* More indie developers will start using MSIX reducing machine rot on user's machines
* Better reputation for "outside" UWP apps

## Scope

| Capability | Priority |
| :---------- | :------- |
| This proposal will allow developers to use MSIX without purchasing an expensive code signing certificate while still being secure | Must |
| This proposal will allow end users to install self-signed packaged apps effortlessly | Should |
| This proposal will allow developers to install self signed apps on user's machine without the user accepting to trust | Won't |