refactor(Core/ServerMail): Refactor to Dedicated Manager Class with Multi-Item & Condition Support (#21590)

1. Core Cleanup
    - Move all ServerMail logic from `ObjectMgr` into a new dedicated `ServerMailMgr` class
    - Move faction logic for money from SendServerMail into the script
2. Separation of items into a new table
    - Create a new `mail_server_template_items` table
    - Allows to send multiple items in one mail
    - Separate items per faction Alliance/Horde
3. Separation of conditions into a new table
    - Create a new `mail_server_template_conditions` table
    - Allows to use multiple conditions for one mail
    - Available condition types
        - Minimum playtime (playerLevel >= condition)
        - Minimum playtime (playerPlayTime >= condition)
        - Rewarded quest
        - Earned achievement
        - Earned reputation (playerReputation >= conditionState)
        - Faction
        - Race
        - Class
4. Updated ServerMail loading
    - Move item and condition loading to their own functions
        - LoadMailServerTemplateItems()
        - LoadMailServerTemplateConditions()
5. Reworked eligibility check
    - Player needs to pass all conditions to be eligible for the mail
    - All players are automatically eligible if no conditions exist for a server mail template.
6. Updated foreign keys
    - For table `mail_server_character`, `mail_server_template_conditions`, `mail_server_template_items` foreign key with on delete cascade is added for automatic removal of entries if mail_server_template.id is deleted.
7. Database changes
    - See the PR

src/server/game/World/World.cpp

@@ -73,6 +73,7 @@
 #include "PoolMgr.h"
 #include "Realm.h"
 #include "ScriptMgr.h"
+#include "ServerMailMgr.h"
 #include "SkillDiscovery.h"
 #include "SkillExtraItems.h"
 #include "SmartAI.h"
@@ -1667,8 +1668,8 @@ void World::SetInitialWorldSettings()
     LOG_INFO("server.loading", "Loading Player Level Dependent Mail Rewards...");
     sObjectMgr->LoadMailLevelRewards();
 
-    LOG_INFO("server.loading", "Load Mail Server Template...");
-    sObjectMgr->LoadMailServerTemplates();
+    LOG_INFO("server.loading", "Load Mail Server definitions...");
+    sServerMailMgr->LoadMailServerTemplates();
 
     // Loot tables
     LoadLootTables();