Add support for 3rd-party symfony mailer transports

Problem/Motivation

Follow on from 📌 [PP-1] Add symfony mailer transports to DIC Postponed . The original issue added the 4 base transports by creating explicit services. Sites that wish to use other transports would need to add extra services. Likely there will be Contrib modules to help with that, however it's awkward because creating the service will fail if the transport isn't installed, and each site will have a different set of transports. For example we could have a sub-module for each transport, containing only a single service definition.

Symfony is aware of a further 14 (at time of writing) 3rd-party transports. These can be accessed using the function Transport::getDefaultFactories(), which returns only the transports that are installed.

Proposed resolution

We can automatically create services for these 3rd-party transports if a service doesn't already exist. This is simple because they all follow exactly the same template like this.

  Symfony\Component\Mailer\Transport\NullTransportFactory:
    parent: Symfony\Component\Mailer\Transport\AbstractTransportFactory
    tags:
      - { name: mailer.transport_factory }

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet