Add ability to register custom item transformers.

[Golfing7]

Information

This PR closes Nothing.

Details

Proposed feature:
A system to allow plugin developers to add custom item transformers to metadata on items. Would be useful for plugins seeking to add custom item templates that could be dynamic or have special data on the item.

Environments tested:

OS: Windows 10

Java version: 1.8.0_281

• Most recent Paper version (1.XX.Y, git-Paper-BUILD)
• CraftBukkit/Spigot/Paper 1.12.2
• [ x ] CraftBukkit 1.8.8

Demonstration:

A demonstration isn't fully applicable to the situation as it's not something that immediately affects anything.

[mdcfe]

Thanks for your interest in contributing to EssentialsX!

As it stands, MetaItemStack has been long in need of a major overhaul which I've been working on in a private branch. Part of this rework would overlap with the functionality added in this PR, so I will need some time to consider whether it would make sense to merge this PR at this stage.

[Golfing7]

It's been a while, just would like to see if this could still be merged as the rework to MetaItemStack appears to have not come yet. Thanks!