Multiple email transport configuration for different purposes

This proposal has been discussed on the Mautic forums at https://forum.mautic.org/t/ability-to-specify-different-email-gateways-to-use-for-different-purposes/10620.)

The feature request has also been tracked in GitHub issue #3827 since 2017 (https://github.com/mautic/mautic/issues/3827) and has generated significant community interest across multiple forum threads.

Currently, Mautic only allows configuration of one email service provider (such as Sendgrid, Mandrill, or Amazon SES) at a time. This creates several critical challenges.

This project will see us implementing, in core, a comprehensive multiple email transport system that allows administrators to:

1. Configure multiple email transports: Add and manage multiple email service providers (Amazon SES, Sendgrid, Mandrill, Mailgun, SMTP servers, etc.) within a single Mautic instance, each with their own credentials and configuration.

2. Email-level transport selection: Assign specific transports to individual emails at the email creation/editing stage, visible in the email settings panel. This allows marketers to designate which service sends which email. 

3. Purpose-based transport assignment: Implement transport selection based on email purpose:
   Transactional emails (form submissions, password resets, notifications)

   Marketing emails (campaigns, segment broadcasts)

   Per-campaign or per-email designation

4. Contact owner-based routing: Enable transport selection based on contact ownership, allowing different teams, brands, or business units to send through their designated providers whilst sharing a single Mautic instance. 

5. Fallback mechanism: Implement a default transport that serves as fallback when no specific transport is assigned to an email.

6. Transport management interface: Create an intuitive interface within Mautic settings to add, edit, remove, and test multiple transports without needing to modify configuration files.

7. Mautic 7+ compatibility: Ensure the solution works with Mautic's Symfony Mailer architecture and DSN-based configuration system.