Add support for custom decoder and encoder

[rsmything]

Parent: https://github.com/braverhealth/phoenix-socket-dart/pull/59

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

[rsmything]

@matehat please check it

[tlvenn]

@Pacane any chance to review and merge this ?

[matehat]

Thanks for your contributions @rsmything ! And sorry for the delay ..!