New villager: Stablemaster

The stablemaster would be a rare villager that spawns in plains villages alongside a new stable building. It would be able to trade you pretamed llamas, mules, donkeys and horses which would be released from the stable building. It could also trade you saddles, leads and horse armour. It could also spawn as a wandering variant in almost any biome with a horse, mule, donkey or llama which it could trade to you. For example, if it appears riding a horse, it will only be able to trade you a horse. Once you have traded for the mob, it will become a professionless villager.