Allow Sender to be set via policy.

Closes #3388651

src/Plugin/EmailAdjuster/SenderEmailAdjuster.php

+<?php
+
+namespace Drupal\symfony_mailer\Plugin\EmailAdjuster;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\symfony_mailer\Address;
+use Drupal\symfony_mailer\EmailInterface;
+
+/**
+ * Defines the Sender Email Adjuster.
+ *
+ * @EmailAdjuster(
+ *   id = "email_sender",
+ *   label = @Translation("Sender"),
+ *   description = @Translation("Sets the email sender header."),
+ * )
+ */
+class SenderEmailAdjuster extends AddressAdjusterBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(EmailInterface $email) {
+    $address = $this->configuration['addresses'][0];
+    $email->setSender(new Address($address['value'], $address['display']));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsForm(array $form, FormStateInterface $form_state) {
+    $form = parent::settingsForm($form, $form_state);
+    // Only one sender is allowed.
+    unset($form['add']);
+    return $form;
+  }
+
+}