Make Java Edition entity models data-driven just like items and blocks

Currently, in Minecraft 26.3, entity models are hard-coded in the game's Java code: this means that we have to work with entity renderers to add layers, edit parts or poses, etc.

By making entity models data-driven, we could:

- Edit model parts / poses, or add layers made in editors like Blockbench, without requiring any sort of mods

- Make entity models more maintainable & shareable

- Have a stable interface for models, as the entity renderer code may change with Minecraft versions

So please, for the love of modders, make entity models JSON files