Feature: #62960 - Signal for mailer initialization

See Issue #62960

Description

This signal allows for additional processing upon initialization of a mailer object, e.g. registering a Swift mailer plugin.

Registering the signal:

$signalSlotDispatcher = \\TYPO3\\CMS\\Core\\Utility\\GeneralUtility::makeInstance(\\TYPO3\\CMS\\Extbase\\SignalSlot\\Dispatcher::class);
$signalSlotDispatcher->connect(
        \\TYPO3\\CMS\\Core\\Mail\\Mailer::class,
        'postInitializeMailer',
        \\Vendor\\Package\\Slots\\MailerSlot::class,
        'registerPlugin'
);

The slot class:

<?php
namespace Vendor\\Package\\Slots;

use TYPO3\\CMS\\Core\\Mail\\Mailer;

class MailerSlot {
        /**
         * @param Mailer $mailer
         * @return void
         */
        public function registerPlugin(Mailer $mailer) {
                // Processing here
        }
}

Impact

Extensions may now perform arbitrary processing for every mail.