Expose BinaryMessenger from FlutterEngine in Windows API

[stuartmorgan-g]

Currently the only way to get a BinaryMessenger is as a plugin. Adding an engine->messenger getter to the C API is simple, but the C++ wrapper will need to be extracted from plugin_registrar.cc so that it can be re-used.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.