[1.21.4] Add model loading plugins to replace ModelEvent.ModifyBakingResult

[XFactHD]

This PR adds model loading plugins which are intended to replace ModelEvent.ModifyBakingResult. These allow more precise model wrapping, including wrapping unbaked models, and support loading custom blockstate-like formats. Another benefit of this API design is that it allows mods which implement on-demand model loading/baking a lot more control and improves compatibility between those mods and ones using the model wrapping APIs.
As is probably very obvious, this PR is very heavily inspired by Fabric API's model loading APIs and therefor has an almost 100% overlap in terms of API surface and injection points.

The model loading plugins provide seven entrypoints into the model loading and baking process:

• BlockStateResolvers: blockstate resolvers are registered for specific blocks and are responsible for providing the state->model mapping for all states of the block they are registered for. This allows implementing custom blockstate-like formats, among other things
• On-load/on-load-block modifiers: on-load modifiers are registered globally and allow wrapping unbaked models and unbaked blockstate models respecitively as they are loaded and therefor affect all further uses of the model in question
• Pre-bake/pre-bake-block modifiers: pre-bake modifiers are registered globally and allow wrapping unbaked models and unbaked blockstate models respecitively right before baking and therefor only affect the specific baking process following their execution
• Post-bake/Post-bake-block modifiers: post-bake modifiers are registered globally and allow wrapping baked models right after baking (effectively equivalent to wrapping models in ModelEvent.ModifyBakingResult)

Apart from the missing documentation, there are still a few questions open:

• BlockStateResolvers provide access to the UnbakedBlockStateModels loaded by vanilla as this allows use cases where all models of a particular block have to be wrapped without having to fall back to checking every single model going through the pipeline in an on-load-block modifier.
  Should this access be limited to models for the states being resolved or should it be able to provide access to any model (I have a use case for this which also relies on the other block's resolver having run) which has the downside of potentially recursive resolution with on-demand loading/baking?
• Should we try to aggressively re-use context objects?
• What would be the preferred way of exposing the plugins to mods implementing on-demand loading/baking?

[neoforged-pr-publishing]

• Publish PR to GitHub Packages

Last commit published: 1530b5bfd43eb5e5b74a74aaa2b85dcb809bd240.

PR Publishing

The artifacts published by this PR:

• 📦 net.neoforged:neoforge:21.4.76-beta-pr-1884-model_loading_plugins
• 📦 net.neoforged:testframework:21.4.76-beta-pr-1884-model_loading_plugins

Repository Declaration

In order to use the artifacts published by the PR, add the following repository to your buildscript:

repositories {
    maven {
        name 'Maven for PR #1884' // https://github.com/neoforged/NeoForge/pull/1884
        url 'https://prmaven.neoforged.net/NeoForge/pr1884'
        content {
            includeModule('net.neoforged', 'neoforge')
            includeModule('net.neoforged', 'testframework')
        }
    }
}

MDK installation

In order to setup a MDK using the latest PR version, run the following commands in a terminal.
The script works on both *nix and Windows as long as you have the JDK bin folder on the path.
The script will clone the MDK in a folder named NeoForge-pr1884.
On Powershell you will need to remove the -L flag from the curl invocation.

mkdir NeoForge-pr1884
cd NeoForge-pr1884
curl -L https://prmaven.neoforged.net/NeoForge/pr1884/net/neoforged/neoforge/21.4.76-beta-pr-1884-model_loading_plugins/mdk-pr1884.zip -o mdk.zip
jar xf mdk.zip
rm mdk.zip || del mdk.zip

To test a production environment, you can download the installer from here.