Support IfDifferent metadata

[stan-sz]

In dotnet/msbuild#8743 MSBuild added support for IfDifferent item metadata. SDK does not support it. Example:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Publish.targets

Lines 892 to 899 in 127f992

	<ItemGroup>
	<_SourceItemsToCopyToPublishDirectoryAlways KeepMetadata="$(_GCTPDIKeepMetadata)"
	Include="@(_NoneWithTargetPath->'%(FullPath)')"
	Condition="'%(_NoneWithTargetPath.CopyToPublishDirectory)'=='Always'"/>
	<_SourceItemsToCopyToPublishDirectory KeepMetadata="$(_GCTPDIKeepMetadata)"
	Include="@(_NoneWithTargetPath->'%(FullPath)')"
	Condition="'%(_NoneWithTargetPath.CopyToPublishDirectory)'=='PreserveNewest'"/>
	</ItemGroup>