[Enhancement] Provide Opacity property for brushes

In our WPF application we're using gradients with their opacity property set to lower values than 1.

It would be great if we'd have this feature in MAUI so we can easily port it.

E.g.
```
<LinearGradientBrush StartPoint="0,0" EndPoint="0,0.5" Opacity="0.2">
    <GradientStop Color="White"/>
    <GradientStop Color="#7F6D6A69" Offset="1"/>
</LinearGradientBrush>
```