Export HttpProtocol

[jenow]

What does this PR do ?

Forgot to export the HttpProtocol class in the main kuzzle.dart file

How should this be manually tested?

import 'package:kuzzle/kuzzle.dart';

void main() {
  final proto = HttpProtocol(
    Uri(
      scheme: 'http',
      host: 'localhost',
      port: 7512,
    ),
  );
}

HttpProtocol should not be unknown.