Add armor trim event and a system to disable events based on feature flags

[bl4ckscor3]

This PR first and foremost adds a new event that applies a random item trim to every armor item that the player is wearing. Armor that exists only in the player's inventory is not affected. Because armor trims are an experimental feature and locked behind the update_1_20 datapack, the event is as well. To make this possible, I added a system for locking events behind feature flags. If an event requires a specific feature flag which is not enabled, it cannot be selected in a vote or at random, and it cannot be run manually via the command.