Merge pull request #8828 from jtattermusch/csharp_call_flags_api

src/csharp/Grpc.Core.Tests/CallOptionsTest.cs

src/csharp/Grpc.Core.Tests/Internal/FakeNativeCall.cs

src/csharp/Grpc.Core/CallOptions.cs

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

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

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

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

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

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

src/csharp/ext/grpc_csharp_ext.c