code: final byte[] task3(AwsSigner4Request sr, AuthConfig credentials) {
          return hmacSha256(getSigningKey(sr, credentials), task2(sr));
      }
docs: Task 3.
  <a href="https://docs.aws.amazon.com/general/latest/gr/sigv4-create-string-to-sign.html">Calculate the Signature for AWS Signature Version 4</a>