include assembly signing key in the repo

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

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

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

.../Grpc.Examples.MathClient/Grpc.Examples.MathClient.csproj

.../Grpc.Examples.MathServer/Grpc.Examples.MathServer.csproj

src/csharp/Grpc.Examples.Tests/Grpc.Examples.Tests.csproj

src/csharp/Grpc.Examples/Grpc.Examples.csproj

...harp/Grpc.HealthCheck.Tests/Grpc.HealthCheck.Tests.csproj

src/csharp/Grpc.HealthCheck/Grpc.HealthCheck.csproj

...ationTesting.Client/Grpc.IntegrationTesting.Client.csproj

...esting.QpsWorker/Grpc.IntegrationTesting.QpsWorker.csproj

...ationTesting.Server/Grpc.IntegrationTesting.Server.csproj

...rp/Grpc.IntegrationTesting/Grpc.IntegrationTesting.csproj

src/csharp/keys/Grpc.snk

src/csharp/keys/README.md