Core/Battlegrounds: Implement mercenary system

Port From (https://github.com/TrinityCore/TrinityCore/commit/87723e32e24f03ac1d062e33edcbe935e4e59421)

Source/Game/Spells/Auras/AuraEffect.cs

@@ -5736,6 +5736,17 @@ namespace Game.Spells
             }
         }
 
+        [AuraEffectHandler(AuraType.SwitchTeam)]
+        void HandleSwitchTeam(AuraApplication aurApp, AuraEffectHandleModes mode, bool apply)
+        {
+            if (!mode.HasFlag(AuraEffectHandleModes.Real))
+                return;
+
+            Player player = aurApp.GetTarget().ToPlayer();
+            if (player != null)
+                player.SwitchToOppositeTeam(apply);
+        }
+        
         [AuraEffectHandler(AuraType.SetFFAPvp)]
         void HandleSetFFAPvP(AuraApplication aurApp, AuraEffectHandleModes mode, bool apply)
         {

Source/Game/Spells/SpellManager.cs

@@ -4338,6 +4338,23 @@ namespace Game.Entities
                 spellInfo.Attributes |= SpellAttr0.UnaffectedByInvulnerability;
             });
 
+            // Horde / Alliance switch (BG mercenary system)
+            ApplySpellFix(new[] { 195838, 195843 }, spellInfo =>
+            {
+                ApplySpellEffectFix(spellInfo, 0, spellEffectInfo =>
+                {
+                    spellEffectInfo.Effect = SpellEffectName.ApplyAura;
+                });
+                ApplySpellEffectFix(spellInfo, 1, spellEffectInfo =>
+                {
+                    spellEffectInfo.Effect = SpellEffectName.ApplyAura;
+                });
+                ApplySpellEffectFix(spellInfo, 2, spellEffectInfo =>
+                {
+                    spellEffectInfo.Effect = SpellEffectName.ApplyAura;
+                });
+            });
+
             foreach (var spellInfo in mSpellInfoMap.Values)
             {
                 // Fix range for trajectory triggered spell