[Request] ParentClass pin on SpawnActorFromClass node

Being able to mark properties as ExposeOnSpawn is a very powerful tool, but it’s completely unavailable if you’re selecting the Actor class at runtime, since the SpawnActorFromClass node reverts to the generic Actor class.

If we had two class pins (the Actor class and the Parent class), we could get the best of both worlds: the Actor class could be dynamic while the Parent class is fixed, allowing the node to display ExposeOnSpawn properties from the Parent class while spawning dynamically selected derived classes.

(Also, some documentation on the workings of the K2Node classes would help us peasants to roll our own nodes).