Add `affecting_items` to Stat endpoint

[DaltonSW]

I think it would be nice to get back a reference to the items that affect a Pokemon's stat, either EV related or in battle

I'm honestly unsure if there would be many more things in it other than the X <Whatever> items, but still wanna throw it out there

[PaoloAJ]

I would like to work on this. I'll keep you updated!