Function fetch 

pub async fn fetch(
    endpoint: &IrohEndpoint,
    remote_node_id: &str,
    url: &str,
    method: &str,
    headers: &[(String, String)],
    req_body_reader: Option<BodyReader>,
    req_trailer_sender_handle: Option<u64>,
    fetch_token: Option<u64>,
    direct_addrs: Option<&[SocketAddr]>,
) -> Result<FfiResponse, CoreError>