NuGet binary

To build the native package, you need precompiled versions of grpc_csharp_ext library artifacts for Windows, Linux and Mac. In the normal gRPC release process, these are built by a Jenkins job and they are copied to the expected location before building the native nuget package is attempted.

See tools/run_tests/build_artifacts.py for more details how precompiled artifacts are built.

When building the native NuGet package, ignore the "Assembly outside lib folder" warnings (the DLLs are not assemblies, they are native libraries).