This change adds Tonic templates for automatically generating bindings for Dart FFI, as well as serialisation of the bindings.

• Adds parallel FfiDispather templates to the existing DartDispatcher used for (old) native bindings.
• Adds serialisation of the bindings to enable automatic conversion and verification.
• Extends existing DartConverters with conversions to and from the FFI transport types.
• Adds new test (ffi_native_unittest.cc) for the above.

This will allow us to replace the existing native functions in e.g. dart:ui with new Dart FFI native functions.

The benefit of FFI calls over the old native calls is that FFI automatically handles data conversion for a number of primitive types like int, double and bool, and thus does not require the overhead of calling Dart API within the native code to deal with Handles.
Additionally, for code that does not use Handles, we can avoid further overhead of setting up frames and transitioning VM state.

With FfiDispatchers in place to set up all the new entry points, the remaining steps to convert the existing native functions to FFI are: 1) setting up the native function resolver, and 2) converting the functions like:

  void moveTo(double x, double y) native 'Path_moveTo';
  /// \/ -- Becomes -- \/
  @FfiNative<Void Function(Pointer<Void>, Float, Float)>('Path::moveTo', isLeaf: true)
  external void moveTo(double x, double y);

The serialisation of the bindings will additionally allow us to automatically generate the above conversions, and subsequently validate all bindings in Dart code match their native counterparts.

Issues:

• Switch all native callbacks in Engine test harnesses to dart:ffi flutter#84379