text: Title: Allow to get CookieJar in IClientService
  Body: With IClientService you can get a wrapped GuzzleClient.
  However you have no way to keep the cookies with our API.
  
  The following would work:
  ```php
  		$cookieJar = new \GuzzleHttp\Cookie\CookieJar();
  
  		$client = $this->clientService->newClient();
  		$response = $client->post(
  			'.../login', [
  				'body' => [
  					'name' => $name,
  					'password' => $password,
  				],
  				'cookies' => $cookieJar,
  			]
  		);
  
  		$response = $client->get(
  			'.../authed-api', [
  				'cookies' => $cookieJar,
  			]
  		);
  ```
  
  I think we should add a `newCookieJar()` method to the clientService, to hide the Guzzle CookieJar. I just don't know what to do with the interface, Should we add a dummy around it like with the client? and forward all calls to the internal?
  
  cc @MorrisJobke @rullzer agreed and okay for 16?