Get the raw "enveloped" message over which the signature for a given input message is computed. This is a low-level function intended for uses cases which can't be expressed using [`SshSig::sign`], such as if the [`SigningKey`] trait can't be used for some reason. Once a [`Signature`] has been computed over the returned byte vector, [`SshSig::new`] can be used to construct the final signature.
(namespace: &str, hash_alg: HashAlg, msg: &[u8])