Function signing_key 

pub fn signing_key(
    datetime: &DateTime<Utc>,
    secret_key: &str,
    region: &str,
    service: &str,
) -> Result<Vec<u8>, String>

Derive the SigV4 signing key.

Computes HMAC(HMAC(HMAC(HMAC("AWS4" + secret, date), region), service), "aws4_request").