Suggestion - Add a reserved PropertyGroup / ItemGroup Type

[JanKrivanek]

Context

If we are ever going to lift the 'sensitive data as a 1st class citizen' topic (https://github.com/dotnet/msbuild/blob/main/documentation/specs/proposed/security-metadata.md) - the nonbreaking way of marking the data type(s) would be handy.

To prepare for this we should consider adding the 'Type' attribute to PropertyGroup / ItemGroup that is accepted, but not interpreted in any way - similarly to the 'Label' attribute:

msbuild/src/MSBuild/MSBuild/Microsoft.Build.Core.xsd

Lines 150 to 154 in a71903d

	<xs:attribute name="Label" type="xs:string" use="optional">
	<xs:annotation>
	<xs:documentation><!-- _locID_text="PropertyGroupType_Label" _locComment="" -->Optional expression. Used to identify or order system and user elements</xs:documentation>
	</xs:annotation>
	</xs:attribute>

This way we could then add the interpretation in future release, while not requiring backward incompatible formats.

Downside - The incompatibility of such marking with old .NET Framework versions of MSBuild would still not be prevented.
However it's very cheap and low risk change, that should be considered even if it's highly likely we won't use it in the future - as they say 'next best time is now'

FYI - @rainersigwald, @baronfel