Add support for streaming CJSeq output

[Ylannl]

So that buildings features are immediately streamed once the reconstruction is complete.

[ymoisan]

Just noting interest in this issue for whenever it fits your roadmap. Rough intent: pick this up as a pure-C++ refactor of CityJsonWriter into a small streaming feature-writer interface, which would also give #147 (option A) a clean seat to land on top of later. Happy to share a fuller sketch and open a draft PR whenever you have breathing room, or to defer entirely if you'd rather keep this one in-house — either is fine.