Add Transform property to Brushes

[amwx]

Is your feature request related to a problem? Please describe.
Allow brushes to be transformed, which WPF, UWP, and WinUI all support
WPF:
https://docs.microsoft.com/en-us/dotnet/desktop/wpf/graphics-multimedia/brush-transformation-overview?view=netframeworkdesktop-4.8

WinUI/UWP have a Transform and RelativeTransform property directly on Brush
https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.media.brush?view=winrt-19041

I've been trying to port the new WinUI styles and they have a couple brushes that have scale transform applied:

 <LinearGradientBrush x:Key="TextControlElevationBorderBrush" MappingMode="Absolute" StartPoint="0,0" EndPoint="0,2">
                <LinearGradientBrush.RelativeTransform>
                    <ScaleTransform ScaleY="-1" CenterY="0.5"/>
                </LinearGradientBrush.RelativeTransform>
                <LinearGradientBrush.GradientStops>
                    <GradientStop Offset="0.5" Color="{StaticResource ControlStrongStrokeColorDefault}"/>
                    <GradientStop Offset="1.0" Color="{StaticResource ControlStrokeColorDefault}"/>
                </LinearGradientBrush.GradientStops>
</LinearGradientBrush>

These brushes are usually used for borders (from what I can tell) so a work around is to separate the border brush into a separate Border element and apply the transform there, but that's not as ideal

Describe the solution you'd like
I think either adding a Transform property to IBrush would work, or use a separate ITransformableBrush interface (naming for example) to just apply to Gradient/Image/etc. brushes since SolidColorBrush probably doesn't need this.