Ability to customize Entity saved alert message

Problem/Motivation

Currently, there is no configuration form field for editing or changing the entity_saved_in_background_alert_message. If we want to make the sentence reframe better to have UI to update the field.

Steps to reproduce

1. Install the module as normal
2. Enable the node content type entity under Allowed Content Entity Forms
3. Select the required content type
4. Edit an existing node and keep in 2 tabs
5. One tab is edited and saved, and another waits to get an alert message.
6. Currently there is no option to edit the message.

Proposed resolution

Add an option for edit the alert message on config form AutosaveFormSettingsForm.php.

    $form['entity_saved_in_background_alert_message'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Entity saved Alert message(background)'),
      '#default_value' => $config->get('entity_saved_in_background_alert_message'),
      '#description' => $this->t("The message will appears on content entity changed or updated."),
      '#required' => TRUE,
    ];

Remaining tasks

Manual testing

User interface changes

Form UI

API changes

Data model changes