Asynchronous loading for ModPrefab

[zorgesho]

Changes made in this pull request

public changes:

• asynchronous virtual method IEnumerator GetGameObjectAsync(IOut<GameObject> gameObject) added to the ModPrefab.
  It is supposed to be used like GetGameObject, only all sync operations with game prefabs should be changed to async (examples will be added later).
• AddPrefabCopy method added to the ModPrefabCache class. It can be used for instantiating prefab and adding that copy to the cache at the same time. That way instantiated copy will not run Awake(), Start() and Update() in both sync and async versions of GetGameObject.

internal changes:

• ModPrefabRequest class added to the SMLHelper.V2.Assets namespace. It is used to get modded prefabs asynchronously (it calls ModPrefab.GetGameObjectInternalAsync)
• various changes in PrefabDatabasePatcher (new patch for object deserialization, using of ModPrefabRequest and some refactoring)

All new code supposed to work on both SN branches. Modders can add GetGameObjectAsync methods to their prefabs along with GetGameObject. For now, on the stable branch, GetGameObjectAsync doesn't do/break anything. And exp branch for now can be used to test GetGameObjectAsync (GetGameObject shouldn't break anything on exp branch).
So mods can be updated to use async prefabs ahead of update and when async update will hit stable branch, mods will not be broken (at least because of that).

[Alexejhero]

I dont think we should merge any PRs which have TODOs.

[PrimeSonic]

This is looking good. I can approve this for now since only minor changes should be pending.