Model load observation API

[embeddedt]

The following PR implements a standardized API mods such as Continuity can use to have low-level control over what model instances are loaded and baked within the ModelLoader. One benefit of standardizing this system is that it will allow optimization mods such as my own to safely implement dynamic model loading/unloading at runtime without running into compatibility problems when mods that inject into ModelLoader directly. It also makes it easy for modders to replace models without needing to learn exactly how the model loader works.

EDIT: Will run checkstyle after other concerns are addressed.

[Technici4n]

This now lives inside of #3145.