code: public String getCrc32cToHexString() {
      if (crc32c == null) {
        return null;
      }
      byte[] decodeCrc32c = BaseEncoding.base64().decode(crc32c);
      StringBuilder stringBuilder = new StringBuilder();
      for (byte b : decodeCrc32c) {
        stringBuilder.append(String.format("%02x", b & 0xff));
      }
      return stringBuilder.toString();
    }
docs: Returns the CRC32C checksum of blob's data as described in <a
  href="http://tools.ietf.org/html/rfc4960#appendix-B">RFC 4960, Appendix B;</a> decoded to
  string.
  
  @see <a href="https://cloud.google.com/storage/docs/hashes-etags#_JSONAPI">Hashes and ETags:
  Best Practices</a>