Added GetAwaiter extension for UnityEngine.Coroutine objects

[Brice-xCIT]

This PR enables awaiting on a coroutine object that has been created previously.

Example:

// start a coroutine
Coroutine coroutine = StartCoroutine(MyCoroutine());

// do things while the coroutine is running.
// ...

// now waits for the coroutine to complete.
await coroutine;

[Ujang360]

@svermeulen what do you think?

[chiuan]

good!