LootList callbacks for Shopkeepers and Fireplaces

[dpower12]

MC_PRE_SHOPKEEPER_GET_LOOT_LIST and MC_PRE_FIREPLACE_GET_LOOT_LIST callbacks to make modded shopkeepers and fireplaces implementations easier with Rgon.

the ability to remove spiders and other drops and replace it with custom ones would help a lot in making new shopkeepers variants with different mechanics attached to them.

[Spearkiller]

Piggybacking off this to also request the same thing but for poops

[namishere]

Poops don't use LootList