Allow adding assembly-level attribute for code generation via MSBuild

[kzu]

Having to add [assembly: GenerateCodeForDeclaringAssembly] is a bit hard to discover, especially when types being used come from nuget packages.

It might be valuable to also offer an MSBuild alternative, just like [InternalsVisibleTo...] can now be done with <InternalsVisibleTo Include=".." /> directly in the csproj.

Something like the following:

<ItemGroup>
   <OrleansGenenerator Include="AssemblyPotentiallyFromPackage" />
</ItemGroup>

This way, it should even be possible for the nuget packages themselves to provide this item automatically so that scenarios that use those types in Orleans context, Just Work.

[kzu]

@ReubenBond if you think this is useful (I do 😉 ), I can work on a PR as outlined in the description.