Be able to specify segment colors for walls

Polylinegeometry allows color arrays to specify the color for each segment
(https://cesiumjs.org/Cesium/Build/Documentation/PolylineGeometry.html)

My suggestion is to add this to walls too
https://cesiumjs.org/Cesium/Build/Documentation/WallGeometry.html

Compare the PolylineGeometry and WallGeometry documentation linked in the issue, starting with how PolylineGeometry accepts per-segment color arrays. Trace the corresponding wall geometry and rendering entry points, then verify that each wall segment can receive its own color and that existing single-color behavior remains intact.

Written by the indexing model from the issue text.