feat: Implement retry for FnMut() -> impl Future<Output=Option<Result<T, E>>>

[Xuanwo]

Future<Output=Option<Result<T, E>>> is widely used in streams that return Result<T, E>. If retry can support this, we can implement a retryable stream as simply as:

stream::unfold(s, move |mut state| {
    async move {
        match { || state.next() }
	        .retry(backoff)
	        .await
        {
            Some(v) => Some((v, state)),
            None => None,
        }
    }
})