Add sync option to execute form

Closes #3443572

src/Form/MigrationExecuteForm.php

@@ -140,6 +140,12 @@ class MigrationExecuteForm extends FormBase {
       '#description' => $this->t('Check this box to update all previously-imported content in addition to importing new content. Leave unchecked to only import new content'),
     ];
 
+    $form['options']['sync'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Sync'),
+      '#description' => $this->t('Check this box to delete all destination content that is no longer present in the source.'),
+    ];
+
     $form['options']['force'] = [
       '#type' => 'checkbox',
       '#title' => $this->t('Ignore dependencies'),
@@ -269,6 +275,7 @@ class MigrationExecuteForm extends FormBase {
     $options = [
       'limit' => $form_state->getValue('limit') ?: 0,
       'update' => $form_state->getValue('update') ?: 0,
+      'sync' => (bool) $form_state->getValue('sync'),
       'force' => $form_state->getValue('force') ?: 0,
     ];