Add support for custom decoder and encoder

[rsmything]

Adding support to add your own encoder and decoder.

For example:

Using BERT decoder for incoming messages https://pub.dev/packages/bert https://github.com/Youimmi/bert.git

import 'package:bert/bert.dart as bert';
import 'package:phoenix_socket/src/message_serializer.dart';

socket = PhoenixSocket(
  socketUrl,
  socketOptions: PhoenixSocketOptions(
    serializer: MessageSerializer(decoder: bert.decode),
  ),
);

[matehat]

Thanks for this!

I'm a little hesitant to loosen the typing of the incoming messages into dynamic because they could be either String or List<int>. What do you think of changing the class hierarchy a bit so that there could be a PhoenixRawSocket of something like that, that expects List<int> and another that expects String?

[rsmything]

Thank you for reply, i will rewrite it soon.