Expand Trial Spawner Registry to a Proper Mob Spawning Registry

In the snapshot versions of Minecraft 1.21.2, the pool of mobs for a trial spawner to pick from when spawning has become data-driven. This means that we can define spawn entries via a JSON file and then reference that file in a trial spawner to have those entities spawn from it. These files are very versatile, allowing us to define how many mobs to spawn at once, how likely each mob is to spawn, the specific NBT data the spawned mob will have, and the equipment it will be wearing, if applicable.

Now compare that with the tools we currently have available for spawning mobs naturally. We have to specify a list of mobs inside of a biome JSON file, meaning there is no way to edit it without overwriting an entire biome definition. The only things we can specify in these lists are the mob ID, the minimum and maximum number to spawn, and the chance of them spawning. We are unable to specify any sort of NBT data or the equipment for them to spawn with. All of these are things that the trial spawner registry can do.

So my suggestion is this: Instead of having an entire registry dedicated to trial spawners, make them a subtype of a larger spawning registry. This would allow much more flexibility when it comes to entity spawning, but would still allow references to trial spawner-specific features. This could also allow the files to be referenced in biome definitions, meaning that data pack creators could change spawning information independently from biome information.