Allow extensions to extend the schema

[martinRenou]

Now that jupytercad-freecad is not in core anymore, the cleanest approach to support all object types from FreeCAD would be to allow it to extend the schema. That way we could add support for Part::FeaturePython etc.

Extensions could:

• add new properties to existing objects
• add new objects definitions
• modify existing properties?
• extends file metadata

We would then need to save in the file the needed extensions for the file to load. e.g. something like the name of Python modules?

"extensions": [
  "jupytercad_freecad",
  "jupytercad_openvsp"
]