Platform channel implementation in C# #127
Description
- PoC for Dart to C# messaging
- Implement Codec and Channel types
- BasicMessageChannel, StringCodec (Add BasicMessageChannel to C# embedding #253)
- StandardMessageCodec (embedding/csharp: Add StandardMessageCodec #266)
- MethodChannel, StandardMethodCodec (embedding/csharp: Add MethodChannel and related classes #267)
- EventChannel (embedding/csharp: Add EventChannel and related classes #270)
- Add a tool support for building C# plugins - maybe?
Note that only primitive types can be supported, similarly to other language implementations and Dart isolates.