Add option to fill only the underrepresented faction (10v10 auto-balance)

[Voidx]

Hi! I’d like to request a feature/setting for CFBG to only fill the faction that is missing players, instead of crossfilling both sides.

Example (10v10):

Alliance queue: 13 players

Horde queue: 9 players

With this option enabled, the system would only add 1 player to Horde (by converting/mirroring from Alliance if needed) so the BG starts as 10v10.
Alliance would not receive converted players since it already has enough.

Goal: reduce unnecessary crossfaction conversions, keep faction identity, and still start BGs faster.

Possible config idea:

CFBG_FillMode = UNDERDOG_ONLY

Thanks!