Create a BC mapper for MailManager to convert to Symfony Mailer

Problem/Motivation

This issue is part of the migration strategy for 🌱 [META] Adopt the symfony mailer component Needs review , related to ✨ Migration strategy for moving to Symfony Mailer Active .

Site builder will wish to start using the new mail system even when some contrib modules don't yet support the new API.

Proposed resolution

1. Create a BC mapper MailManagerReplacement that implements MailManagerInterface, converting to call the new MailerInterface.
2. The main work of the conversion is done using a mailer plug-in LegacyMailer.
3. Use a service provider to replace the 'plugin.manager.mail' service when the new mail service is enabled.

Similar code already exists in Drupal Symfony Mailer module, using LegacyEmailBuilder. The conversion appears to work well for many cases, however it is best-effort given that the $message interface is free-form. The conversion won't handle non-standard uses.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet