Horse Armor Expansion

[Shadows-of-Fire]

Just musing about a few things that would make horse armor more extensible.

1. Delegate Horse#setArmorEquipment to HorseArmorItem so that armor can apply more attribute modifiers than just the armor modifier. a. Vanilla logic has to be replicated, and a method for removal needs to exist as well so modifiers can be cleaned up when the armor is replaced.
2. Delegate Horse#isArmor to IItemExtension instead of requiring p_30731_.getItem() instanceof HorseArmorItem. (Depends on the first point, as currently that method casts to HorseArmorItem).