Function pss_sign_msg 

pub fn pss_sign_msg<H: Hasher>(
    sk: &RsaSecretKey,
    msg: &[u8],
    s_len: usize,
    rng: &mut dyn FnMut(&mut [u8]),
) -> Option<Vec<u8>>

Convenience: hash msg with H, then sign with a random salt.