Method sign

Sign the given message with the provided signing key. See also: [`PrivateKey::sign`]. # Errors - Returns [`Error::AlgorithmUnsupported`] if this signing key's algorithm is unsupported by this library with its enabled features. - Returns [`Error::Crypto`] in the event of an error occurring in the cryptographic implementation. - Returns [`Error::Namespace`] if the namespace is empty.

(
        signing_key: &S,
        namespace: &str,
        hash_alg: HashAlg,
        msg: &[u8],
    )