[WinUI] Implement `SetSemanticFocus` for WinUI

### Description

The `SetSemanticFocus` API in the new SemanticExtensions for accessibility currently has no Windows implementation.

A starter solution is in the works, but the nuances between touch focus and keyboard focus are still being investigated.

### (Public) API Changes

# Semanticextensions

| API | Description |
| ------------- | ------------- |
| SetSemanticFocus | [SetFocus](https://docs.microsoft.com/en-us/windows/winui/api/microsoft.ui.xaml.automation.peers.automationpeer.setfocus?view=winui-3.0) among others still being investigated |

### Usage Scenarios

https://github.com/dotnet/maui/blob/e690737dd81a9014b63680da1c3cc206919d3f1f/src/Controls/samples/Controls.Sample/Pages/Core/SemanticsPage.xaml.cs#L32

### Backward Compatibility

XCT also will need to updated accordingly

### Difficulty

Medium