Add Update-AppInstallerApp: trigger update checks for .appinstaller-installed apps

[shmuelie]

Summary

Add an Update-AppInstallerApp cmdlet that triggers an update check for apps installed from a .appinstaller file, by re-registering against the app installer file (Add-AppxPackage -AppInstallerFile <uri>).

Details

• Complements enumeration of .appinstaller-sourced apps (see the related Get-AppInstallerApp proposal): given an app (or all such apps), kick off its .appinstaller update check.
• Accepts a specific app (by name/identity) or updates all discovered .appinstaller apps.
• Should support -WhatIf/-Confirm since it changes installed app state.

Acceptance criteria

• Triggers an update check for a named app, or for all .appinstaller-sourced apps.
• No-op (not error) when there are none / nothing to update.
• Honors -WhatIf.
• Windows-only; degrades cleanly elsewhere.

Notes

Ships the mechanism only; personal wiring stays downstream.