[B+C] Add EntitySpawnEvent. Adds Bukkit-1559

##### The Issue: #####
Most entity spawns cannot be tracked with events, and polling is required to track new entities.
##### Justification: #####
Polling the server with background loops is heavy on performance. The proper way to check for spawning in Bukkit is through CreatureSpawnEvent, but this only handles LivingEntity.
##### PR Breakdown: #####
Adds a new event called EntitySpawnEvent, which tracks all entity spawns not already handled by events (LivingEntity, Item, and Projectile).
##### Testing: #####
Test plugin here: https://github.com/kenzierocks/TestPluginBukkit1559/
It tracks all entity spawns via the new event, and logs them to the console. Use /cancel-spawns to toggle canceling the event.
##### Relevant PRs: #####
CB: https://github.com/Bukkit/CraftBukkit/pull/1354
##### Ticket: #####
https://bukkit.atlassian.net/browse/BUKKIT-1559
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