Support for color specific letters

[Belzedar94]

Following the discussion from Discord, in some cases, NNUE is not able to properly handle asymmetric variants because the same piece types can have very different roles in the game, depending on the side. The perfect example is Horde.

We did a workaround to define Horde as an alternative variant where the white pawns are custom pawns so that NNUE can have different inputs for black/white pawns. Doing that, the net progressed 200 elo further than the current strongest net for Horde.