code: def make_subdomain_txt(name_or_fqn, domain, address, n, zonefile_str, privkey_bundle):
      
      subrec = Subdomain(str(name_or_fqn), str(domain), str(address), int(n), str(zonefile_str), None, None, None, None, None, None)
      subrec_plaintext = subrec.get_plaintext_to_sign()
      sig = sign(privkey_bundle, subrec_plaintext)
      
      subrec = Subdomain(str(name_or_fqn), str(domain), str(address), int(n), str(zonefile_str), str(sig), None, None, None, None, None)
      return subrec.serialize_to_txt()
docs: Make a signed subdomain TXT record, to be appended to a (domain's) zone file.
      Return the TXT record string