[MAPS3D-1802] Add building-gen submodule and some refactoring (#3552)

Part of https://mapbox.atlassian.net/browse/MAPS3D-1802

This PR does two things:

- Refactors some of the routines and classes previously defined in
`fill_extrusion_bucket.cpp/hpp`.
- Adds https://github.com/mapbox/building-gen as a new submodule. Note
that the new library isn't used in code in this PR.

These are both later used by
mapbox/mapbox-sdk#3522 and allow for a neater
review process.

cc @mapbox/sdk-ci

GitOrigin-RevId: 6ecb8c22d165e7778342f67910be51ad6ffc6aef

Author: akoylasar

LICENSE.md

@@ -1805,6 +1805,16 @@ For more information, please see
 
 ---
 
+### building-gen,1.0.0,ISC,Mapbox,https://github.com/mapbox/building-gen
+
+```
+Copyright (c) 2025 Mapbox
+
+All rights reserved.
+```
+
+---
+
 ### mapbox-base-internal,dev,BSD-3-Clause,Mapbox,https://www.mapbox.com/
 
 ```