Improve ARIA label configuration fields in entity_form_steps formatter schema

Problem/Motivation

The entity_form_steps formatter currently does not provide any way to define ARIA labels for its action buttons (Next, Previous, Submit, Preview, Delete).
As a result, all multi-step form buttons generated by the formatter rely exclusively on their visible text label.

Why this is a problem

1. Screen readers often require more descriptive text than what is displayed visually
   The visible labels used in multi-step forms are typically short (“Next”, “Previous”, “Submit”).
   While appropriate visually, these labels do not provide sufficient context for users relying on assistive technologies.

   Example:
   A screen reader user may not know what they are moving to — "Next" could refer to the next page, the next section, or the next field group.

2. WCAG 2.2 recommends providing clear, descriptive, and contextual labels
   ARIA labels allow developers to provide richer instructions without visually cluttering the interface.

   For example:

   • aria-label="Go to the next step"
   • aria-label="Return to the previous step"
3. The module exposes UI configuration for button labels but does not allow accessible alternatives
   Site builders can change the visible button text, but cannot define an accessibility-specific label.
   This forces site builders to either:
   • Accept reduced accessibility, or
   • Overload the visible button text with excessive detail, harming usability.
4. This limitation negatively impacts users with visual impairments or cognitive disabilities
   A lack of ARIA labels makes it harder to understand:
   • What each button does
   • Where the user is moving within the multi-step process
   • The overall structure of the form

Motivation for the enhancement

Adding configurable ARIA labels for each button provides a significant accessibility improvement by enabling:

• Clearer and more descriptive labels for screen readers
• Better compliance with WCAG 2.2 AA
• Improved usability for users with assistive technologies
• Project-specific tailoring of button descriptions
• Better multilingual support
• More inclusive multi-step workflows

The enhancement maintains full backward compatibility and introduces no functional breaking changes.
It simply adds optional configuration fields that greatly improve the accessibility of forms created using the module.