Generate classes/writers/readers for custom glTF extensions

Hi, I'm wondering what would be the best way to add support for custom glTF extensions.

I suppose I could fork your [glTF repository](https://github.com/CesiumGS/glTF) and add my schema files there, and then also fork cesium-native add reference them in glTF.json - but that complicates things a bit more than I had hoped. Are there any other ways, or would you consider adding support for a custom extension folder and config file, so that I could call:
```
node index.js --schema [SAME PARAMS AS USUAL]... --customExtensions myProject/my_extension --customExtConfig myglTF.json
```

Thanks for reading!

**Update:** Ok, so while the generated JsonHandlers are output as separate files, that's not the case for the Json-writer code - which are a collection of "writeJson" functions inside [ModelJsonWriter.cpp](https://github.com/CesiumGS/cesium-native/blob/main/CesiumGltfWriter/generated/src/ModelJsonWriter.cpp)
So I guess this means that the only way of adding custom extensions would be to fork these two repositories then, right?

Start with node index.js and the schema-generation flow, then inspect the generated JsonHandlers and CesiumGltfWriter/generated/src/ModelJsonWriter.cpp. Determine how a custom extension folder and config would be incorporated without forking the repositories. Done means custom glTF schemas generate the required readers and writers through the proposed command-line options.

Written by the indexing model from the issue text.