Add GUI Registry pre/post hooks

[MattSturgeon]

Overview

This PR adds the ability to register pre & post hooks with the GuiRegistry. Registered hooks are run before and after each Field is transformed into GUIs, allowing users to (for example) get a reference to the final GUIs.

Motivation

A user could achieve a similar effect using a GuiTransformer, however there is a risk that another transformer may "pull the rug out" and remove/replace the GUI in question. Hooks solve this issue with an immutable GUI list and not using a return value. Therefore a post hook is guaranteed to run after all destructive changes have happened.

Without this PR, a user could register their own requirements using the transformer API, which will work for most cases.

With this PR, a user can register requirements without having to make any assumptions.

This PR also starts the ball rolling for a dedicated AutoConfig Requirements API using declarative annotations.

Question

The above motivation hopefully justifies having a "post hook" API, however I've only included a "pre hook" for completeness.
Is there any reasom to include or not include a "pre hook" in this PR? If not, should "post hook" be renamed?

Related changes

This PR includes a number of small related changes to cleanup and modernize GuiRegistry's internal implementation. Namely:

• Use a record for provider/transformer entries
• Use EnumMap instead of HashMap
• Simplify GuiRegistry.get()
• Simplify GuiRegistry.transform()

These changes could be dropped or moved to a different PR, however I felt it was simpler to include them here as they lay the foundation for this PR's core changes.

You may find it easier to review this PR one commit at a time.

A temporary "example" commit is also included, to aid in testing & reviewing. This commit should be dropped before merging (d789d6c).

Original description

Autoconfig requirement support will require operating with finished Config Entry GUIs in order to a) keep track of what GUIs have been created in a lookup table and b) register GUIs that have dependencies declared with a RequirementManager.

This could be done internally by injecting additional code into GuiRegistry or by adding default GuiTransformers, however the former seems short-sighted and the latter isn't very robust, seeing as there's no guarantee the gui list won't change (or be entirely replaced) by another transformer running after us.

IMO the best solution is a "post" event hook. The handler can be passed an Unmodifiable List, ensuring that the GUI objects the handler sees are the same ones that will actually be used.

It seems silly to do this entirely internally, and strange to have a public "post" hook without a "pre" hook, so this PR follows the above to its logical conclusion and adds an experimental public API to register pre & post event hooks on GuiRegistry.

I wasn't sure whether or not it made sense for event hooks to be subject to predicates (in the same way that registered GuiProvider and GuiTransformers are). This can be added easily if desired, but will increase the number of new experimental API methods on GuiRegistry a lot...

This PR is blocking Requirement support for autoconfig (as per #195 & #204).

[MattSturgeon]

d789d6c (debug prints) is included in this PR for demonstrative purposes only. It shouldn't actually be merged.

For additional example usage, you could refer to my in-progress autoconfig-dependencies branch:

public class DefaultGuiHooks {
    public static GuiRegistry apply(GuiRegistry registry) {
        
        registry.registerPostHook((guis, i18n, field, config, defaults, guiProvider) ->
                guiProvider.getLookupTable().register(field, guis));
        
        registry.registerPostHook((guis, i18n, field, config, defaults, guiProvider) ->
                guiProvider.getRequirementManager().registerRequirements(guis, field));
        
        return registry;
    }
}

[MattSturgeon]

I've pushed some changes and clarified the PR description.