Add HTTP Protocol

[jenow]

What does this PR do?

Add HTTP Protocol using https://docs.kuzzle.io/core/2/guides/main-concepts/api/#json-query-endpoint Update flutter sdk.

How to test

import 'package:kuzzle/kuzzle.dart';
import 'package:kuzzle/src/protocols/http.dart';

void main() async {
  final proto = HttpProtocol(
    Uri(
      scheme: 'http',
      host: 'localhost',
      port: 7512
    ),
  );
  final kuzzle = Kuzzle(proto);
  await kuzzle.connect();
  final res = await kuzzle.query(
    KuzzleRequest(
      controller: 'server',
      action: 'now'
    ),
  );
  print(res);
}