update (core): Knock Back Helpers (#12639)

Adds knock back helpers to the core for future core and module use.

src/server/game/Entities/Player/Player.h

@@ -2430,6 +2430,8 @@ public:
 
     bool CanTeleport() { return m_canTeleport; }
     void SetCanTeleport(bool value) { m_canTeleport = value; }
+    bool CanKnockback() { return m_canKnockback; }
+    void SetCanKnockback(bool value) { m_canKnockback = value; }
 
     bool isAllowedToLoot(Creature const* creature);
 
@@ -2883,6 +2885,7 @@ private:
     bool m_bMustDelayTeleport;
     bool m_bHasDelayedTeleport;
     bool m_canTeleport;
+    bool m_canKnockback;
 
     std::unique_ptr<PetStable> m_petStable;