Add a few new `ItemModelProvider` methods!

[ZestyBlaze]

Since ItemModelProvider only provides basicItem which does exactly what it says on the tin, when having to do other items that require something a bit different, such as handheld items or any item that supports a template from Minecraft or your own mod, it requires the developer to write a custom method just to handle these item models or hand write them This PR adds two new methods at the moment, handheldItem and spawnEggItem, however this PR will be marked as draft for a while awaiting community input for more devs to suggest other useful methods they might like to see :)