Fix balloon animals for 26.3

[misode]

Fixes and re-enables the balloon animals for 26.3

Instead of the previous proposed solution where we'd be using vanilla trade JSON files, this takes a different approach:

• After a wandering trader spawns in, the custom trades are still picked, and llamas are replaced with animals
• But the Offers NBT is not yet modified, because that would prevent the vanilla trades from appearing
• Instead the trades are stored in data.gm4_balloon_animals.trades
• And they are copied to Offers after being interacted with using an advancement trigger

Problems with this approach:

• The first time the player opens the trader UI, the ballon animal trades are not visible, and the player has to leave and reopen the UI

[runcows]

Reviewed code and tested in game. Works as expected.

I would suggest adding villager trade unlocked particles on interact to better signal to the player what is happening. But that's not blocking to this