add method
returns true on success otherwise false. has to pass the AtContact to add new contact into the contact_list if atSign value is 'null' then returns false
Implementation
@override
Future<bool> add(AtContact contact) async {
var atSign = contact.atSign;
//check if atSign is 'null'
if (atSign == null) return false;
var atKey = _formKey(KeyType.contact, key: atSign);
var json = contact.toJson();
var value = jsonEncode(json);
return await atClient!.put(atKey, value);
}