Function string_to_sign 

pub fn string_to_sign(
    datetime: &DateTime<Utc>,
    region: &str,
    canonical_req: &str,
    service: &str,
) -> String

Build the string-to-sign for AWS SigV4.

Format: AWS4-HMAC-SHA256\n<ISO8601>\n<scope>\n<canonical_request_hash>