Allow overriding the Soldier Class a unit is promoted to

[robojumper]

XComGameState_Unit.RankUpSoldier should contain a hook to allow mods to override the soldier class instead of having the class be determined by a variety of sources (XComHQ, ResistanceHQ etc.).

This could allow things such as a proper implementation of training roulette by just creating new soldier classes on the fly, provided a) the Unit state itself and b) the NewGameState is passed into the hook.
The return value should be able to indicate that the unit shouldn't be promoted in case someone wants to use this for Commander's Choice.

It should be a X2DLCInfo hook because rank ups can happen in a start state, which does not play well with event listeners.