Function sign_request 

pub fn sign_request(
    key: &SigningKey,
    node: &str,
    audience: &str,
    method: &str,
    path: &str,
    body: Vec<u8>,
    ts_ms: u64,
    nonce: &str,
) -> LicenceRequest

Sign a request with the steward key. nonce must satisfy valid_nonce.