Add DllSafeSearchPathAttribute by default to all .NET Core assemblies

[vatsan-madhavan]

Can we consider adding the following attribute to all .NET Core assemblies by default.

[assembly:DefaultDllImportSearchPathsAttribute(DllImportSearchPath::AssemblyDirectory | DllImportSearchPath::System32)]

BuildTools already seems to do this here:

• Initializes IncludeDllSafeSearchPathAttribute property here
• Generates the assembly attribute here

Assemblies that are now generated in an Arcade SDK based repo seem to be silently losing this assembly level attribute.

cc @ericstj, @rladuca, @stevenbrix, @dotnet/wpf-developers