Officially support downloading signed appx packages from the Store for distribution via own website

[benstevens48]

Description of the new feature/enhancement

Presumably, in order to install apps from the Store, winget uses a publicly accessible API to download the signed app packages. This proposal is that the API should be officially documented. The API should include the ability to download the latest signed packages from specified package flights (as well as the unlighted version), and possibly also specified package versions.

The use-case is for developers to be able to distribute the signed app packages via their own website. Crucially, this would allow developers to offer old versions of apps or different branches such as LTS, current and beta, by creating an AppInstaller file and uploading the packages to their own website. (Currently there is no way to do this with the Store, except by collecting a limited number of users email addresses to add to a package flight, which is impractical). This is especially useful for apps with addon purchases, since it is very difficult (currently) to think of a way of distributing an app outside the Store while maintaining access to addon purchases. The Store would still receive a cut of addon purchases as they would continue to work in the normal way.

Ideally there would be an even more friendly way to download the packages and create an AppInstaller file, but the API would be a first step.

For further discussion on this issue, please see microsoft/WindowsAppSDK#39, and also let me know if anything in their warrants a new issue here. Also, if the idea of being able to install old versions of apps (in a user-friendly way) is being addressed via some other means, please let me know, and then maybe this issue is obsolete.

Proposed technical implementation details (optional)

This is actually already possible, it's just not officially supported - see the comment here microsoft/WindowsAppSDK#39 (comment).

Ideally, an app installed via an AppInstaller file outside the Store would not be automatically upgraded via the Store. However, this can be worked around (somewhat hackishly) by ensuring versions intended to be installed outside the Store have a very large package version number, so the unflighted package version number will always be lower.

[RokeJulianLockhart]

#578 (comment)

@denelon, where is this documented?