Make CriteriaTriggers an IForgeRegistry

Title is not entirely accurate, but the point is that currently the CriteriaTriggers for advancements has a private REGISTRY and register() method.  Without being able to register a new ICriterionTrigger object with this registry, modders cannot create new advacement triggers (this includes recipes that award on something other than the vanilla triggers).  For example, I want to create a trigger that will test true when any one of a list of items is picked up (the vanilla InventoryChanged trigger does take a list, but it needs all of them to be acquired before being true) as I have six flower blocks that are all basically the "same" block (limits on metadata based state information) and I want to award an advancement if any one of those six blocks is acquired (technically possible with vanilla triggers, if I smashed all six ItemBlocks into a single Item--yuck). I then have another advancement I want to trigger when the player uses bonemeal and grows any one of these flowers (which not possible with vanilla triggers).

I have no idea how things need to be set up to fire registry events and so forth, otherwise I would do it myself as a PR. Presumably these would need to be registered before recipes and advancements but after item models.

(The ICriterionTrigger classes are also insanely complicated, involving two static child classes and most of the one I have was pieced together from existing vanilla versions).