Issue #3274069 by AdamPS, Jaypan: Add interfaces

modules/symfony_mailer_bc/src/MailManagerReplacement.php

...symfony_mailer_bc/src/MailManagerReplacementInterface.php

src/EmailFactory.php

src/Entity/MailerPolicy.php

src/Entity/MailerPolicyInterface.php

src/MailerHelper.php

src/Processor/EmailAdjusterManager.php

src/Processor/EmailAdjusterManagerInterface.php