Adds StandardMethodCodec support to the C++ client wrapper. This makes it
substantially easier to add Windows and Linux support for existing plugins, as
StandardMethodCodec is the default plugin protocol.
Fixesflutter/flutter#30670
Does not include extensibility for the codec, which will be added later.
Adds a type that can hold any of the types corresponding to the Dart types
that are supported by the standard message channel codec. This provides
the foundation for adding standard message codec support for the C++
desktop shells (flutter/flutter#30670).
Changes include:
- File structure
- Header guards
- Include paths
- Namespaces
- Integration with the engine's GN build
- Conversion from jsoncpp to rapidjson
- Style and clang-format adjustment to match engine repository
