Show AJAX progress indicator for save/cancel actions

Fount similar problem in LayoutParagraphsBuilderForm::buildForm(). This form related to the layouts management, I'm going to add progress indicator to both forms within this issue. I'm not sure about message property, so leaving it empty for now.

Checked commits history and there no obvious reasons to keep 'progress' => 'null'

For context, panchuk's fix here is inspired by a similar update that was made in https://www.drupal.org/project/layout_paragraphs/issues/3265669 → . progress was previously set to none there as well, and it appears to have been just a minor oversight rather than an intentional choice. Perhaps the same is true here?