add custom serializer to match GeoJSON spec

[vincentsarago]

closes #125

[eseglem]

I like the idea, but have been trying to think through all the implications. A bit worried about ignoring any of the serialization kwargs that are used, and I don't entirely know how things are handled when nested due to not having much documentation on it. Especially when it comes to how Properties will be handled.

I managed to get this to work. It calls the built in serializer then removes the field from the result.

    @model_serializer(when_used="json", mode="wrap")
    def clean_model(self, serializer: Any, _info: SerializationInfo) -> Dict[str, Any]:
        """Custom Model serializer to match the GeoJSON specification."""
        data: Dict[str, Any] = serializer(self)
        if "bbox" in data and data["bbox"] is None:
            del data["bbox"]
        return data

Which led me to attempting to make it more generic, and a rabbit hole of additional generalization... I'll probably throw up a PR soon to see what you think.