Picker Styling: Borders

### Description

I would love the ability to have border styling across all platforms for the picker, instead of having to manually add the mappings ourselves

### Public API Changes

So far, I have only really done a mapping on Windows, when I make a PR I will investigate the other platforms

## Windows Example

```csharp
PickerHandler.Mapper.AppendToMapping("Border", (handler, view) =>
{
if (handler.PlatformView is ComboBox comboBox)
{
comboBox.BorderBrush = new SolidColorBrush(Windows.UI.Colors.Red);
comboBox.BorderThickness = new Microsoft.UI.Xaml.Thickness(2);
}
});

```

### Intended Use-Case

Further customisation without having to write platform-specific code

Start at the PickerHandler.Mapper mapping shown in the issue and inspect the picker handler mappings for each platform. Compare the existing Windows example with the other platform implementations, then verify that a public border styling option works consistently across platforms without manual platform-specific mappings.

Written by the indexing model from the issue text.