Pass entity object to MailManager as param.

Problem/Motivation

As describe in #3016953, it would be nice to be able to use hook_mail_alter to make changes to the email based on some values/conditions of the entity. Currently the entity object isn't available in hook_mail_alter though, making this difficult.

Proposed resolution

Pass the entity object to the $this->mailManager as a param so it is available inside hook_mail_alter. This appears to be what the core Contact module does.

Should be possible by changing /src/Plugin/QueueWorker/WorkbenchEmailProcessor.php to add the entity that is already available.

// Send the email.
=$this->mailManager->mail('workbench_email', 'template::' . $template->id(), $data->getTo(), LanguageInterface::LANGCODE_DEFAULT, [
    'body' => $body,
    'template' => $template,
    'subject' => $subject,
    'entity' => $entity,
], $replyTo);

Remaining tasks

1. Agreement on feature request.
2. Make patch to add functionality.

User interface changes

None

API changes

None

Data model changes

None