Decouple Golems from Players and use SpawnMonster for Golem

[obligaron]

Contributes to #543 and supports #7663

In master and vanilla golems are tied to a specific monster id. This monster id corresponds 1:1 to the player id. This is how the relation player<->golem relationship is maintained and that's also why the game reserves the first 4 monster slots for golems (max player count in multiplayer).

This PR changes that by

• using goalVar3 to maintain the golem<->player relationship (should be 0 in singleplayer saves)
• introducing a helper to find golems for a player
• using SpawnMonster logic to spawn golems (similar to how monsters are spawned dynamically, see Support dynamically spawned monsters in multiplayer #6890)
• to maintain vanilla compatibility, the first 4 slots are still reserved for golems. In singleplayer, the first monster slot is always used.

Additional notes

• Allows multiple golems to be spawned by removing the KillGolem call in AddGolem for existing golems (allows better mods).
• This PR supports up to 55 players because we still store the enemy id in a uint8_t (could be changed later).
• This PR can be tested with the current player count by changing ReservedMonsterSlotsForGolems to a lower number (I tested with 2 players and set it to 0 and 1).