[Windows] Improve AOT compatibility on Windows

[simonrozsival]

Description of Change

• We had several places where we suppressed the IL2059 on Windows. This isn't needed anymore because the generated code in XamlTypeInfo.g.cs is trim safe.
• We don't need .rsp files as a workaround anymore. The bug in the XamlPreCompile step has been fixed in WinUI.
• We were using AOT-incompatible Marshal.SizeOf(typeof(T)). This can be easily fixed by using the generic overload.
• We were using trim-incompatible JSON serialization. This can be easily replaced with source-generated JSON serialization.
• We need to make quite a few classes partial to make the CsWinRT com-wrappers source generator happy.
• We need to allow unsafe blocks because the CsWinRT source generator produces unsafe code.

This is a follow-up to #24266, cc @Foda