minor #16373 [Mailer] Add custom transport factories (AlbertMorenoDEV)

This PR was submitted for the 4.4 branch but it was squashed and merged into the 5.4 branch instead.

Discussion
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[Mailer] Add custom transport factories

This might be useful for very custom use cases where we need to extract some data or do something before being able to create the actual transport.

The feature apparently was added here: symfony/symfony#31946

Commits
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72b69c1 [Mailer] Add custom transport factories

Author: javiereguiluz

mailer.rst

@@ -356,6 +356,54 @@ Other Options
 
         This option was introduced in Symfony 5.2.
 
+Custom Transport Factories
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There is a way to easily create your own custom transport factory in case you need to do something special creating the actual transport.
+
+The new factory needs to implement :class:`Symfony\\Component\\Mailer\\Transport\\TransportFactoryInterface`. To remove some boilerplate you can even extend from :class:`Symfony\\Component\\Mailer\\Transport\\AbstractTransportFactory` which will simplify the new factory::
+
+
+    final class CustomTransportFactory extends AbstractTransportFactory
+    {
+        public function create(Dsn $dsn): TransportInterface
+        {
+            // create and return the transport
+        }
+
+        protected function getSupportedSchemes(): array
+        {
+            return ['custom_schema'];
+        }
+    }
+
+Finally, declare the new factory in setting tag the tag `mailer.transport_factory`:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/services.yaml
+        mailer.transport_factory.custom:
+            class: App\CustomTransportFactory
+            parent: mailer.transport_factory.abstract
+            tags:
+              - {name: mailer.transport_factory}
+
+    .. code-block:: xml
+
+        <!-- config/services.xml -->
+        <service id="mailer.transport_factory.custom" class="App\CustomTransportFactory" parent="mailer.transport_factory.abstract">
+            <tag name="mailer.transport_factory"/>
+        </service>
+
+    .. code-block:: php
+
+        // config/services.php
+        $services->set('mailer.transport_factory.custom', App\CustomTransportFactory::class)
+                 ->parent('mailer.transport_factory.abstract')
+                 ->tag('mailer.transport_factory');
+
 Creating & Sending Messages
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