renaming stub to client and refactoring metadata class

src/csharp/Grpc.Auth/OAuth2InterceptorFactory.cs

src/csharp/Grpc.Core/Calls.cs

src/csharp/Grpc.Core/ClientBase.cs

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

src/csharp/Grpc.Core/Internal/MetadataArraySafeHandle.cs

src/csharp/Grpc.Core/Metadata.cs

src/csharp/Grpc.Core/Stub/StubConfiguration.cs

src/csharp/Grpc.Core/Version.cs

src/csharp/Grpc.Examples.MathClient/MathClient.cs

src/csharp/Grpc.Examples/MathExamples.cs

src/csharp/Grpc.Examples/MathGrpc.cs

src/csharp/Grpc.HealthCheck/HealthGrpc.cs

src/csharp/Grpc.IntegrationTesting/TestGrpc.cs