Add in default transport factories

Problem/Motivation

Symfony\Component\Mailer\Transport::getDefaultFactories() returns all the transport factories that are available on the system. In v1 we had code that called it.

In v2 I was copying #3379794: Add symfony mailer transports to Dependency Injection Container (mail delivery layer), however the feature was deferred to #3501138: Add support for 3rd-party symfony mailer transports. So we need to add the equivalent code back into this module.

Proposed resolution

Create MailerTransportServiceProvider that registers the default factories if they don't already exist.

Remaining tasks

User interface changes

API changes

Data model changes