Initial migration to new .csproj files

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Auth/Grpc.Auth.project.json

src/csharp/Grpc.Auth/Grpc.Auth.xproj

src/csharp/Grpc.Auth/packages.config

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.csproj

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.project.json

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.xproj

src/csharp/Grpc.Core.Testing/packages.config

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.csproj

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.project.json

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.xproj

src/csharp/Grpc.Core.Tests/packages.config

src/csharp/Grpc.Core.Tests/project.json

src/csharp/Grpc.Core/Common.csproj.include

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.Core/Grpc.Core.project.json

src/csharp/Grpc.Core/Grpc.Core.xproj

src/csharp/Grpc.Core/NativeDeps.Linux.csproj.include

src/csharp/Grpc.Core/NativeDeps.Mac.csproj.include

src/csharp/Grpc.Core/NativeDeps.Windows.csproj.include