Remove dependency on UniTask

[jasonboukheir]

Instead of using UniTask to handle asynchronous code, state machines implementing IEnumerator should be used for a low-level API. This allows developers to handle the waiting logic themselves however they want to, without requiring 3rd-party dependencies.

If developers want to use unitask, they can, by using the IEnumerator. If developers want to use coroutines, they can.