feature #33968 [Notifier] Add Firebase bridge (Jeroeny)

This PR was squashed before being merged into the 5.1-dev branch (closes #33968).

Discussion
----------

[Notifier] Add Firebase bridge

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | See #33687
| License       | MIT

This would add [Firebase](https://firebase.google.com) integration for the Notifier component. With Firebase you can send push notifications to the users of you Android and iOS app and website (formerly known as Google Cloud messaging).

I'm not sure if it's possible to have this merged, like the other bridges. Or if I should create a stand-alone repository? That'd be fine too.

Also it's now using the `ChatMessage` as implementation of `Symfony\Component\Notifier\Message\MessageInterface`, but I feel like this component could use a `PushMessage` or something similar. Although I'm not sure if it would contain more than `subject` that the `ChatMessage` does.

Commits
-------

2776d2f811 [Notifier] Add Firebase bridge

Author: fabpot

Exception/UnsupportedSchemeException.php

@@ -46,6 +46,10 @@ class UnsupportedSchemeException extends LogicException
             'class' => Bridge\Twilio\TwilioTransportFactory::class,
             'package' => 'symfony/twilio-notifier',
         ],
+        'firebase' => [
+            'class' => Bridge\Firebase\FirebaseTransportFactory::class,
+            'package' => 'symfony/firebase-notifier',
+        ],
     ];
 
     /**

Transport.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Notifier;
 
+use Symfony\Component\Notifier\Bridge\Firebase\FirebaseTransportFactory;
 use Symfony\Component\Notifier\Bridge\Mattermost\MattermostTransportFactory;
 use Symfony\Component\Notifier\Bridge\Nexmo\NexmoTransportFactory;
 use Symfony\Component\Notifier\Bridge\RocketChat\RocketChatTransportFactory;
@@ -42,6 +43,7 @@ class Transport
         NexmoTransportFactory::class,
         RocketChatTransportFactory::class,
         TwilioTransportFactory::class,
+        FirebaseTransportFactory::class,
     ];
 
     private $factories;