RP base model config

[BPR02]

This PR adds a new config option to the resource pack to modify the base item model definition. This is specifically useful when finer control over the base item model is needed. Previously this functionality was not available and with our resource pack workflow, certain features were not implementable. This PR allows the entire "model" JSON object to be modified.

E.g. to acheive the following custom model data:

{
  "threshold": 3420002,
  "model": {
    "type": "minecraft:model",
    "model": "gm4_scuba_gear:item/flippers",
    "tints": [
      {
        "type": "minecraft:dye",
        "default": -14455863
      }
    ]
  }
}

The following base_model config can be used:

model_data:
  - item: leather_boots
    reference: item/flippers
    template: generated_overlay
    base_model: 
      tints: 
        - type: minecraft:dye
          default: -14455863

Or alternatively with JSON:

model_data:
  - item: leather_boots
    reference: item/flippers
    template: generated_overlay
    base_model: {
      "tints": [
        {
          "type": "minecraft:dye",
          "default": -14455863
        }
      ]
    }

[SpecialBuilder32]

Looks pretty simple and straightforward to me.