feat: Surprise bag boss

[Laslest]

This pull request adds support for marking surprise bag loot as exclusive to boss monsters and improves the way surprise bag loot is rolled and distributed. It introduces a new bossOnly attribute for bag items, updates the loot-dropping logic to respect this attribute, and refactors the loot rolling mechanism for better maintainability and Lua scripting support.

Surprise Bag Loot Improvements:

• Added a bossOnly attribute to bag items in bags.xml and the item system, allowing certain surprise bag drops to be restricted to boss monsters only. (data/items/bags.xml, src/items/items.hpp, src/items/items.cpp)
• Refactored the surprise bag loot rolling logic into a new method rollSurpriseBagLoot, which filters and rolls bag loot based on monster type, class, race, and the new bossOnly flag. (src/items/items.cpp, src/items/items.hpp)
• Updated the monster loot drop process to use the new rollSurpriseBagLoot method, ensuring surprise bag drops are handled consistently and with the new restrictions. (src/creatures/monsters/monster.cpp)

Lua Scripting Enhancements:

• Exposed the new surprise bag loot rolling to Lua via MonsterType:getSurpriseBagLoot(), allowing scripts to query possible surprise bag drops for a monster. (src/lua/functions/creatures/monster/monster_type_functions.cpp, src/lua/functions/creatures/monster/monster_type_functions.hpp)
• Integrated the new Lua method into the monster loot generation pipeline, so Lua-based loot logic can now include surprise bag loot. (data/libs/functions/monster.lua)