Allows the use of async/await (instead of yield) with any Unity AsyncOperation

UnityAsyncOperationAwaiter.cs

public static class ExtensionMethods
{
    public static TaskAwaiter GetAwaiter(this AsyncOperation asyncOp)
    {
        var tcs = new TaskCompletionSource<object>();
        asyncOp.completed += obj => { tcs.SetResult(null); };
        return ((Task)tcs.Task).GetAwaiter();
    }
}

/* Example:
var getRequest = UnityWebRequest.Get("http://www.google.com");
await getRequest.SendWebRequest();
var result = getRequest.downloadHandler.text;
*/

This does not seem work with UnityWebRequest when the request fails (i.e. AsyncOperation.isDone is set but AsyncOperation.completed is not invoked). In this case it makes the Task and awaiting routines hang indefinitely.

This probably can't be solved without continuously checking the AsyncOperation.completed flag somehow?

Thank you a lot for this! I've been having a headache with making Untiy wait for a web request to complete before doing something in a function. This helps solve it, again thanks!