Allow CustomModelData for parent models

Currently, CustomModelData only works directly on the json model that the item uses. So carrot's CustomModelData only changes based on what carrot's json model defines. This means that any overrides from parent models are completely ignored, and also any overrides nested in overrides are ignored.

So you can have a CustomModelData that then has more CustomModelDatas in it's own model.

Where this would be useful is if we wish to override lots of child models in the same way.

One way I wanted to use this was to change the block/block model, to add an override for when CustomModelData is 1000, it would be flat texture in the GUI, instead of a 3D block. I wanted this as a CustomModelData so I could have both.

However, because block/block is used as a parent model, all of it's child models, and therefore, all block models, ignore this override completely.

This also happens with override models.

You can have nested overrides, where a CustomModelData model has extra overrides in it's own json file, they all have to be on the original model.

The only downside to this would be it would be possible to create endless loops, however, that would only happen if the resourcepack models are poorly structured.