FFI asynchronous callback samples

We might want to add a sample for asynchronous callbacks to this repo. This requires compiling against `dart_api(_dl).h`.

See the samples in the SDK repo:

* Dart code: https://github.com/dart-lang/sdk/tree/master/samples/ffi/async
* C code: https://github.com/dart-lang/sdk/blob/master/runtime/bin/ffi_test/ffi_test_functions_vmspecific.cc

Context: https://github.com/dart-lang/sdk/issues/46135

Start by reading the Dart and C async FFI samples linked from the issue, along with the referenced Dart SDK issue 46135. Identify how those samples compile against dart_api(_dl).h and adapt the example for this repository. Done means the repository contains an asynchronous callback sample that builds and demonstrates the documented behavior.

Written by the indexing model from the issue text.