Add ability to define custom cache boosters

[gitoido]

Is your feature request related to a problem? Please describe.
Sometimes i want to use something else instead of lapis for cache boost.

Describe the solution you'd like
Ideally i want to be able to configure eligible items in rad_gyms_server.json config in this way:

{
  "cache_boosters": {
    "minecraft:lapis_lazuli": 1,
    "minecraft:lapis_lazuli_block": 9,
    "minecraft:diamond": 18
  }
}