Core/Spawn: Make SpawnGroup respect the pool

[Killyana]

Description

If you make a spawn group and put members inside a pool, once the spawn group is activated it will spawn every member and ignore the pooling.
Adding the support for pooling will allow us to randomize entities inside the spawngroup and make quests like "triage" https://www.wowhead.com/wotlk/quest=6624/triage https://www.youtube.com/watch?v=TdMOERz3pAs easy to script.

Expected behaviour

Spawn Group must respect the pooling instead spawning all members.

Steps to reproduce the problem

1. Use this sql Quest: Triage  #25387 (comment)
2. .go c id 12939
3. Take the quest to start the script
4. Notice in each spot there's 3 spawns on top of each other instead of 1 (in each spot there's 3 pooled spawns)
   Note: After a while when npcs start dying only one npc will respawn at once, so only the initial spawn call must be fixed.

Branch

3.3.5

TC rev. hash/commit

Rev. d3a38dc

Operating system

Windows 11 x64

Custom changes

None