Remove hard-coding referring to PsiOperative class in XComGameState_Unit and Allow other classes to be treated as Psionic

[kyuubicle]

There are several mods on the workshop that recreate the Psi-Operative as a new class in order to allow them to rank up like normal units. However, the Chosen (and likely other things) react to specifically the PsiOperative class. One mod (which seems to have been removed by the author) fixed this by overriding several functions in XCOMGameState_Unit

Several functions in XComGameState_Unit function refer to PsiOperative class specifically in order to do things such as the Hair-Coloring and Random Perk Allocation (RankUpSoldier), and Preventing the soldier from ever gaining AP or ranking up normally (HasAvailablePerksToAssign).

Another function, IsPsionic(), checks for this class as well. As does IsPsiOperative()

Ideally, a fix would allow a unit to be treated as a Psionic-Class (PsiOperative) in all respects and allowing the PsiOperative or other such class to gain AP and rank up normally.
IE: Separating being Psionic/PsiOperative from ranking up normally, gaining AP normally, and assigning Perks normally.

[BTernaryTau]

A hook for overriding the output of HasAvailablePerksToAssign() can be taken from the vanilla highlander: https://github.com/X2CommunityCore/X2CommunityHighlander/pull/202/files

[robojumper]

Ideally, a fix would allow a unit to be treated as a PsiOperative in all respects and allowing the PsiOperative to gain AP and rank up normally.

This is worded very ambiguously. If it gains AP and ranks up normally, it by definition isn't treated like a Psi Operative in all respects.

[kyuubicle]

Sorry, I meant in all other respects the PsiOperative is treated as... a PsiOperative while having the ability to gain XP, Ranks, and assign perks normally.
IE: Separating being Psionic/PsiOperative from ranking up normally and gaining AP normally.

[Onidotmoe]

Many places around the code it simply checks if the soldier's classname is Psi Operative, changing those to ex: use the bHasPsiGift property that's already define in the codebase could be a nice way to remove this weird check.

bHasPsiGift is not used anyways.

[Iridar]

To catch up on this issue.

There were a lot of attempts to address it.

#604 suggested that any unit with eStat_PsiOffense > 0 should count as psionic, the PR was even merged, but then reverted, because it breaks with LWOTC.

#673 suggested to add a configurable list of soldier class templates that count as psionic classes, it was merged, but then reverted, as it had issues keeping its scope coherent.

The only actually merged PR on this theme is #733, which adds a configurable list of soldier class templates that can equip Psi PCS.

Personally I think the modern approach would be to standardize all checks in base game code to XCGS_Unit::IsPsiOperative() and then add a CHL event to let mods decide which units count as psionic based on any arbitrary logic.

I would also revert #733, as it was not publicly released yet, so there is no opportunity to break backwards compatibility, and let any unit that passes a IsPsiOperative() check equip Psi PCS.

[Iridar]

Per Xymanek's recommendation, adding this to 1.22 milestone so this doesn't get lost. This needs to be done in the next immediate Highlander version, while there is still time to revert #733.

Something to actually replace #733 can be added later, though.