feat: add Twig helper generating hub URLs and setting authorization cookies

[dunglas]

This PR dramatically improves the Developer eXperience of the Mercure integration in Symfony. It can be considered as part of the Symfony UX initiative.

At a glance, it introduces a new Twig helper allowing to generate the URL of the Hub and to automatically set the cookie required to use the authorization mechanism:

<script>
    const es = new EventSource(`{{ mercure('https://example.com/users/1', {
        subscribe: ['https://example.com/users/{id}', 'https://example.com/books/{isbn}']
    }) }}`);
    es.onmessage = console.log;
</script>

In a single call, this helper:

1. returns the URL of the Mercure hub with the properly encoded topic query parameters (currently, passing the URL of the Hub to Twig requires some collaboration from a controller, or to define a global environment variable, and encoding the topics may not be straightforward for users not familiar with modern JavaScript APIs)
2. sets a mercureAuthorization cookie containing the valid JWT to subscribe to topics matching the passed URI templates (currently, setting this cookie require collaboration from the controller, and isn't straightforward)

Subscribing to Mercure updates can be considered as part of the View layer. This new helper allows subscribing directly from Twig, without having to write any boilerplate code.

Then, a call to HubInterface::publish() from any service or controller is enough for the client to receive the push!

<?php

namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Mercure\HubInterface;
use Symfony\Component\Mercure\Update;
use Symfony\Component\Routing\Annotation\Route;

class MercureController extends AbstractController
{
    #[Route('/publish', name: 'publish')]
    public function subscribe(HubInterface $hub, Request $request): Response
    {
        return new Response(
            $hub->publish(new Update('https://example.com/users/1', $request->get('data')))
        );
    }
}

This will also allow improving the developer experience of Symfony Turbo by having native support for authorization.

Here is the list of all available options:

mercure(['https://example.com/users/1', 'https://example.com/books/abc'], {
    subscribe: ['https://example.com/users/{id}, 'https://example.com/books/{isbn}'],
    publish: ['https://example.com/users/{id}, 'https://example.com/books/{isbn}'],
    hub: 'another-hub'
})

I also revamped a bit #60 (cc @norkunas) to leverage the new infrastructure. In a following PR, I'll use this to allow to automatically clear the cookie when the user logs out from the site.

PR for MercureBundle: symfony/mercure-bundle#57

TODO:

• add more tests

[weaverryan]

This is a very compelling idea :)