Items can use more than name to pass their original id to client for resource packs

[mfishma]

Adds an opt-out configuration (pass-original-item-name-to-resource-packs) that allows 1.21.4+ clients to clearly identify downgraded items beyond relying on fragile custom_name matching.

When an item is downgraded (e.g., minecraft:copper_axe), ViaBackwards appends its original unmapped item identifier to the strings array in the minecraft:custom_model_data component. By putting it here rather than completely overriding the item_model component, this ensures zero disruption to existing server resource packs or logic.

To prevent polluting server-bound packets, a temporary injected_cmd_string tracking tag is added to the custom_data payload, allowing ViaBackwards to strip any injected string before forwarding the item packet back to the server.

Resource pack development

Resource packs can now natively target downgraded items using vanilla minecraft:select conditions on the custom_model_data property, completely ignoring localized names or user renaming.

Example iron_axe.json fallback definition:

{
  "model": {
    "type": "minecraft:select",
    "property": "minecraft:custom_model_data",
    "index": 0,
    "cases": [
      {
        "when": "minecraft:copper_axe",
        "model": { "type": "minecraft:model", "model": "myresourcepack:item/copper_axe" }
      }
    ]
  }
}

Note: There is potential conflict with other mods that might use custom_model_data. When using modpacks and custom items, server owners should base them off items that were in the game before 1.21.4.

[mfishma]

Discovered one issue. If an item's mapped twice through the downgrade process, both items are injected, and we just want one. I'll fix that before submitting again.