Add support for vertical tabs in remote forms

[dontub]

With this change a \Civi\RemoteTools\Form\FormSpec\VerticalTabsContainer with \Civi\RemoteTools\Form\FormSpec\FormTab as elements can be used to add vertical tabs to a remote form. A \Civi\RemoteTools\Form\FormSpec\FormTab is a container for any form element.

systopia-reference: 26546

[jensschuppe]

Not exactly sure what the Vertical Tabs concept corresponds to in each implementation (e.g. the concept exists in Drupal, but maybe not others), but being able to explicitly define it seems a good idea.