Proposal: allow ProgressBar's background to be customable when Indeterminate=true

# Proposal: allow ProgressBar's background to be customable when Indeterminate=true

## Summary

According to code here: https://github.com/microsoft/microsoft-ui-xaml/blob/589ced40542b9b4de2fddf0e58a1e3eecd164e2f/src/controls/dev/ProgressBar/ProgressBar.xaml#L98C1-L99C1, ProgressBar control's track is hidden when it's in Indeterminate mode, which makes `Background` property invalid.

It would make more sense if users were allowed to set whether the background track is visible in both modes.

## Rationale

* Background property becomes invalid in Indeterminate mode, and there is no way to find this unless referring to the XAML code.
* It is unnecessarily harder to customize the ProgressBar control when UI designs have a visible background track.

## Scope

## Important Notes

## Open Questions

Start with the referenced ProgressBar.xaml section at src/controls/dev/ProgressBar/ProgressBar.xaml#L98-L99 and read the linked feature proposal process. Clarify the API and behavior needed to make the background track configurable in both determinate and indeterminate modes; done would be a resolved proposal with scope and implementation details.

Written by the indexing model from the issue text.