Function with_retries 

pub(crate) async fn with_retries<F, Fut, T, E>(
    cfg: &RetryCfg,
    deadline: Option<Instant>,
    op: F,
) -> Result<T, E>
where
    F: FnMut() -> Fut,
    Fut: Future<Output = Result<T, (Retry, E)>>,

Retry op with exponential backoff. Retry::Permanent returns at once; Retry::Transient retries until the tries run out or deadline passes, with sleeps truncated to it.