Allow prefab instances created in the editor to be part of a pool at game start

[ettmetal]

Currently, there is no way to manually pre-place an object from a pool in a scene, meaning that pooled objects can only be spawned via scripting. It might be nice to have the capability of pre-positioning some objects which will be pooled once the game is running.

[ettmetal]

I think the smoothest way to implement this for now is an editor tool to 'spawn from selected pool'. Maybe a context menu item, a button in the pool's inspector, or a window to drag from. This gets around needing to know about prefab relationships at runtime (not possible).

It would be nicer to have the prefab, when added to the scene, be added automagically 🧙‍♂️ to the appropriate pool. I imagine that would require an OnHierarchyChanged handler that would check known pools for prefab similarity, then inject a PooledObject script with the required info. This might take a bit more work, working with the prefab systems.