I’m surprised to hear that your ArcGIS WMS service only supports 102100. Usually ArcGIS can reproject on the fly. Also, you might try using ArcGisMapServerImageryProvider instead of the WMS provider.
To answer your actual question, though, Cesium itself supports WebMercator imagery, it’s only the WMS imagery provider that does not. Fortunately, it’s easy to work around by editing the code. Change this line:
to construct a WebMercatorTilingScheme instance instead of the GeographicTilingScheme. Don’t forgot the update the list of requires at the top of the file. If you’re not using a global extent for the provider, you’ll need to convert them to WebMercator units (WebMercatorProjection.project can help with this) and pass extentSouthwestInMeters and extentNortheastInMeters parameters to the constructor instead of the extent parameter. Then, change this line to specify the new SRS in the URL:
If you haven’t built Cesium yourself before, it’s not hard. Follow the instructions in the Contributor’s Guide;
I hope this helps!