Core/Entities: Add Unit::IsValidAssistTarget and Unit::IsValidAttackTarget functions for proper serverside checks of related entity flags.

src/server/game/Entities/Player/Player.cpp

@@ -20612,6 +20612,7 @@ void Player::UpdateHomebindTime(uint32 time)
 void Player::UpdatePvPState(bool onlyFFA)
 {
     // TODO: should we always synchronize UNIT_FIELD_BYTES_2, 1 of controller and controlled?
+    // no, we shouldn't, those are checked for affecting player by client
     if (!pvpInfo.inNoPvPArea && !isGameMaster()
         && (pvpInfo.inFFAPvPArea || sWorld->IsFFAPvPRealm()))
     {