[pigeon] Add FFI support to C++

[xiaowei-guan]

Some of our platform channel code is generated using Pigeon, and we can support Pigeon in generating FFI code.
Refer to: flutter/packages#11352

[tarrinneal]

Hey! Let me know if I can support you guys with this somehow. If there are structural changes that can make it easier for you to implement this feature, I will be happy to make them.

[xiaowei-guan]

Hi @tarrinneal,

Thank you so much for your generous offer to help — we really appreciate it!

Actually, our current work is built directly on top of your work (flutter/packages#11352). It's been a great foundation for us to start from.

We're currently still in the investigation/research phase, but we'll definitely reach out if we run into anything where structural changes on your end could make things easier. Thanks again for your willingness to support us!