This makes two changes:
- Adds a way to register a callback for when a FlutterDesktopPluginRegistrarRef is destroyed, and implements the logic to call it in the Windows and Linux embeddings.
- Adds a class to the C++ wrapper that handles making a singleton owning PluginRegistrar wrappers, and destroying them when the underlying reference goes away, to avoid needing that boilerplate code in every plugin's source.
Fixes https://github.com/flutter/flutter/issues/53496
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.
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
