Grpc.Tools should hook CoreCompile instead of BeforeCompile for WPF compatibility

[uecasm]

Using Grpc.Tools 2.27.

Protobuf file generation does not work correctly in WPF; this is listed as a known issue for .NET Core but does also affect .NET Framework as well.

The problem is that Grpc.Tools is hooking into the wrong part of the build process -- in particular a part that does not get executed for a particular compilation step used by WPF.

See dotnet/wpf#810 (comment) for more info.

The suggested fix is to change _Protobuf_Compile_BeforeCsCompile to hook CoreCompile instead of BeforeCompile.

I have not checked if this resolves the issue with .NET Core as well, although I would assume so. Defining the extra property (as shown in the comment above) definitely does resolve it for .NET Framework, and changing the BeforeTargets should have the same effect, but I have not explicitly tested that either.

[uecasm]

And now I have tested a patched nuget package which does just change the BeforeTargets from BeforeCompile to CoreCompile, and confirmed that this works for .NET Framework.

[JamesNK]

Please recreate this issue in https://github.com/grpc/grpc - Grpc.Tools is part of that repository.

[uecasm]

Ok, done. (FWIW I posted it here due to #574.)