Add coven witch to spawn shop

[Eralyen]

Since Witchery's coven witches are generated at worldgen they're really hard to find by now, and they're essential for many of its endgame rites.

If their spawn eggs were added to spawn it would help a lot, since as far as I can tell the only alternative is to roam around the overworld for days (a maxed coven is made up of 6 witches).

Spawn shop already has a few hard to find mob eggs like the ocelot's, so I don't think coven witches would be out of place.