revamp C# native library loader

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

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

src/csharp/Grpc.Core/GrpcEnvironment.cs

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

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

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

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

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

...csharp/Grpc.Core/Internal/ChannelCredentialsSafeHandle.cs

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

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

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

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

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

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

...arp/Grpc.Core/Internal/NativeMetadataCredentialsPlugin.cs

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

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

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

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