New Model API improvements

[LambdAurora]

I've finally got around and tried the API, I liked some parts of it, but I've found some shortcomings.

First of all, I'd really appreciate a very simple ModelLoadingPlugin.Context#addModel(Identifier, UnbakedModel) method.
Having to first add an identifier then wait for the model resolver to come is quite annoying, especially when you need to load block state dependent models that can't go through the model resolver nor the block state model resolver!

It's also currently impossible to tell the ModelLoader to load additional blockstates files for a same block (useful in some composite models!).

Hoping someone can get around to look into those.