Scheduled Tasks

# Proposal: MSIX manifested Scheduled Tasks

## Summary
Provide a way to define [Scheduled Tasks](https://learn.microsoft.com/windows/win32/taskschd/task-scheduler-start-page) via `AppxManifest.xml`, similar to `windows.startupTask` and other extensions.

Microsoft-internal task [36388362](https://task.ms/36388362)

## Rationale

MSIX has no equivalent:

* `windows.startupTask` only fire at logon.
* `windows.preinstallConfigTask` only fires on first install (and with caveats)
* `windows.updateTask` only fires on updates
* Background Tasks can't be defined in `AppxManifest.xml` (require runtime call to BackgroundTaskBuilder)
* Background Tasks are cumbersome for non-UWP (require helper UWP process, or COM server in a long-running process)

## Scope
| Capability | Priority |
| :---------- | :------- |
| Support a `windows.scheduledTask` adding/updating a Scheduled Task when a package is registered for a user | Must |
| Support a `windows.scheduledTask` removing a Scheduled Task package is deregistered for a user | Must |
| Support a `windows.scheduledTask` running as the user the package is registered for | Must |

## Important Notes

N/A

## Open Questions

N/A