Change ICustomLoader interface

[JangkyuSeo]

Purpose of this PR

Change ICustomLoader interface to make it easier to edit

Before

        public interface ICustomLoader
        {
            public AsyncOperationHandle<GameObject> CustomLoad(object key);
            public void CustomUnload(AsyncOperationHandle<GameObject> handle);
        }

After

        public interface ICustomLoader
        {
            public void CustomLoad(string key, Action<GameObject> loadDoneAction);
            public void CustomUnload(string key);
        }

---

Release Notes

Change ICustomLoader interface to make it easier to edit

---

Testing status

Manual

---

Overall Product Risks

Technical Risk: Low

Halo Effect: Low

---

Comments to reviewers