[Feature Request]: Allow using multiple sources simultaneously in the same run.

The request

At least 1 of the following:

• Have WAU scan all locally enabled sources by default.
• Add an "all sources" option to WAU.
• Allow specifying 2 or more $source-s, though I can see this one would be complicated to implement.

Is your feature request related to a problem?

With the third-party source [winget-extras](https://github.com/pl4nty/winget-extras) slowly (as in, *slowly*) making its mark on the wider Winget project, it has gradually become clear to me that the default configs of WAU only runs checks for the main `winget` source.

And I can't find anything in the .ps1 files that allow specifying more than 1 source per run, or any sort of "all sources" options.

To the extent it seems able to change the source, it only allows using 1 source at a time. So if I enabled the `winget-extras` source in WAU, it'd disable the `winget` source.

Additional information

Instance screenshot of updates that are not detected by WAU's default config at the time of writing:

I tried client-side to see if removing $Source and similar strings from the .ps1 files worked for this, but it caused the update check to get stuck at the The 'msstore' source requires that you view the following agreements before using. part.

This issue report is probably of particular interest to @pl4nty.

Inspect the repository's .ps1 files to trace how the $Source value is selected and passed to Winget, then reproduce the behavior with both winget and winget-extras enabled. Define the result as one run checking multiple enabled sources without disabling the default source or getting stuck on the msstore agreement prompt.