custom method
Implementation
String custom({required DateTime expiresIn, required Map payload}) {
final header = _generateHeader();
final aPayload = _generatePayload(
payload
..addAll({
'iat': payload['iat'] ?? dateToSeconds(DateTime.now()),
'exp': dateToSeconds(expiresIn),
}),
);
final signature = _generateSignature(header, aPayload);
return '$header.$aPayload.$signature';
}