feature #57903 [Mailer] Implement Postal mailer (jonasclaes)

This PR was squashed before being merged into the 7.2 branch.

Discussion
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[Mailer] Implement Postal mailer

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| License       | MIT

This will implement the Postal mailer.

API docs: https://apiv1.postalserver.io/controllers/send/message.html

Docs: symfony/symfony-docs#20088

Commits
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db84ab3021 [Mailer] Implement Postal mailer

DependencyInjection/FrameworkExtension.php

@@ -2642,6 +2642,7 @@ private function registerMailerConfiguration(array $config, ContainerBuilder $co
             MailerBridge\Mailomat\Transport\MailomatTransportFactory::class => 'mailer.transport_factory.mailomat',
             MailerBridge\MailPace\Transport\MailPaceTransportFactory::class => 'mailer.transport_factory.mailpace',
             MailerBridge\Mailchimp\Transport\MandrillTransportFactory::class => 'mailer.transport_factory.mailchimp',
+            MailerBridge\Postal\Transport\PostalTransportFactory::class => 'mailer.transport_factory.postal',
             MailerBridge\Postmark\Transport\PostmarkTransportFactory::class => 'mailer.transport_factory.postmark',
             MailerBridge\Resend\Transport\ResendTransportFactory::class => 'mailer.transport_factory.resend',
             MailerBridge\Scaleway\Transport\ScalewayTransportFactory::class => 'mailer.transport_factory.scaleway',

Resources/config/mailer_transports.php

@@ -22,6 +22,7 @@
 use Symfony\Component\Mailer\Bridge\Mailjet\Transport\MailjetTransportFactory;
 use Symfony\Component\Mailer\Bridge\Mailomat\Transport\MailomatTransportFactory;
 use Symfony\Component\Mailer\Bridge\MailPace\Transport\MailPaceTransportFactory;
+use Symfony\Component\Mailer\Bridge\Postal\Transport\PostalTransportFactory;
 use Symfony\Component\Mailer\Bridge\Postmark\Transport\PostmarkTransportFactory;
 use Symfony\Component\Mailer\Bridge\Resend\Transport\ResendTransportFactory;
 use Symfony\Component\Mailer\Bridge\Scaleway\Transport\ScalewayTransportFactory;
@@ -57,6 +58,7 @@
         'mailpace' => MailPaceTransportFactory::class,
         'native' => NativeTransportFactory::class,
         'null' => NullTransportFactory::class,
+        'postal' => PostalTransportFactory::class,
         'postmark' => PostmarkTransportFactory::class,
         'resend' => ResendTransportFactory::class,
         'scaleway' => ScalewayTransportFactory::class,