custom MarkupExtension are not supported XamlReader

[Xiaoy312]

Current behavior

XamlReader.Load("""
	<Border xmlns... Background="{utu:ResponsiveExtension ...}" />
""");

attempting to parse custom MarkupExtension will result in this exception:

Object of type 'Uno.Toolkit.UI.ResponsiveExtension' cannot be converted to type 'Microsoft.UI.Xaml.Media.Brush'.

because we are simply not making custom handling rule for MarkupExtensions in XamlObjectBuilder‎::ProcessMemberElements
and assign it directly to the dependency property

Expected behavior

No response

How to reproduce it (as minimally and precisely as possible)

class ResponsiveExtension : MarkupExtensions { }

XamlReader.Load("""
	<Border xmlns... Background="{local:ResponsiveExtension}" />
""");

Workaround

No response

Works on UWP/WinUI

None

Environment

Uno.WinUI / Uno.WinUI.WebAssembly / Uno.WinUI.Skia

NuGet package version(s)

Uno.WinUI@5.1.0-dev.486

Affected platforms

WebAssembly, Android, iOS, Skia (WPF)

IDE

Visual Studio 2022

IDE version

No response

Relevant plugins

No response

Anything else we need to know?

You also must fully name the markup extension, can't skip the "-Extension" suffix