RadialGradientBrush should expose AlphaMode setting (WCT compat)

Describe the bug One thing we did when we added the RadialGradientBrush to the Windows Community Toolkit is add an AlphaMode property from the underlying Win2D systems. This allowed for finer control of the brush settings, especially for transparency. We left the defaults for WPF compatibility of course.

Screenshots From Toolkit sample app:

Default behavior:

Premultiplied:

Version Info

NuGet package version: v2.4.0-prerelease.200422001

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop
Mobile
Xbox
Surface Hub
IoT

Additional context

[chingucoding]

Left this comment on the PR to depricate it in WCT, leaving this here too for clarity:

The WinUI brush uses the CompositionRadialGradientBrush which does not expose this property (or anything comparable). Maybe somebody else knows more about this, but I don’t think that the current implementation would be able to provide an AlphaMode property.

[michael-hawker]

@chingucoding exactly, usually most of the heavy lifting on these items is done in the initial feature work. For instance, I imagine adding this last property should be pretty straight-forward for RadialGradientBrushes case (famous last words). 😋