feat: item model overrides

[Roelymole]

This is the groundwork for fixing #351.

The top_item_override property can be be included in the data object with the texture resource location to be used for the top face of the machine when it is in item form and also in the side config tab, but not when it is placed in the world.

{
  "data": {
    "back": "galacticraft:block/machine_side",
    "front": "galacticraft:block/advanced_solar_panel",
    "left": "galacticraft:block/machine_side",
    "right": "galacticraft:block/machine_side",
    "top_item_override": "galacticraft:block/solar_panel"
  },
  "machinelib:type": "machine"
}

The item_override property can be provided with a model resource location to be used to override the entire item model, which is not preferable as this would not display the configured sides of the machine, however I have left this in as it might be useful for something and there was a comment saying it was on the todo list.

{
  "data": {...},
  "machinelib:type": "machine",
  "item_override": "namespace:item/overriding_model"
}

I have not currently allowed for other faces to be overriden with front_item_override, left_item_override, bottom_item_override, etc. because I could not think of a valid use case at the moment, however it is a simple enough matter to add these later if need be.