Add support for HTTP tunnelling

[leemaguire]

• Add support for HTTP tunnelling.

    realm::proxy_config proxy_config;
    proxy_config.port = 8080;
    proxy_config.address = "127.0.0.1";
    proxy_config.username_password = {"username", "password"};

    realm::App::configuration app_config;
    app_config.proxy_configuration = proxy_config;
    auto app = realm::App(app_config);

    auto user = app.get_current_user();
    auto sync_config = user->flexible_sync_configuration();
    sync_config.set_proxy_config(proxy_config);
    auto synced_realm = experimental::db(sync_config);