Function retrieve_user 

pub fn retrieve_user<C, Fut, B, E>(
    client: C,
    params: &Params,
) -> RetrieveUser<Fut, B, E> 
where
    C: FnOnce(Request<String>) -> Fut,
    Fut: Future<Output = Result<Response<B>, E>>,
    B: Body,

Retrieve user