Make generators configurable in map JSON files

[Genau6502]

This PR adds a generator field to all map config files. This allows fine-tuning of spawn chances and speeds of items in all generators, allowing for per-map balancing. This approach means that the overall increasing of rates in #41 is not needed - small maps can be tweaked to have slower generators and vice versa

Default generator speed as of before this PR:

"generator": {
    "level_1": {
      "gold": 2,
      "iron": 10,
      "spawn_interval_ticks": 34
    },
    "level_2": {
      "gold": 2,
      "iron": 10,
      "spawn_interval_ticks": 30
    },
    "level_3": {
      "gold": 9,
      "iron": 36,
      "emerald": 1,
      "spawn_interval_ticks": 26
    },
    "diamond": 1,
    "diamond_spawn_interval": 900,
    "emerald": 1,
    "emerald_spawn_interval": 1800
  }

[Gegy]

Ooh yay- thank you for this PR! This is a very much needed change, very excited to see what we can do with this new configuration in terms of balancing different maps!