Move handling of selection settings and route to a #process callback of the form element

Problem/Motivation

Currently, the field widget implementation (Drupal\autocomplete_deluxe\Plugin\Field\FieldWidget\AutocompleteDeluxeWidget) contains code to handle the selection settings (store them in key value store) and add the route with its parameters on the element.

By doing so:

• it's difficult to override certain selection settings from custom code - e.g. in a hook_field_widget_single_element_form_alter() implementation - without having to duplicate a lot of code (store new selection settings in the key value store, set the route again with the right parameters)
• it requires duplicating the same code when trying to use the autocomplete_deluxe form element as a standalone element in a form, outside of any entity fields

Proposed resolution

Move parts of the code related to selection settings & route with its parameters from the widget implementation to the form element implementation, similar to how it is done in Drupal core's entity autocomplete form element. See Drupal\Core\Entity\Element\EntityAutocomplete::processEntityAutocomplete().

Remaining tasks

• ✅ File an issue
• ➖ Addition/Change/Update/Fix
• ➖ Testing to ensure no regression
• ➖ Automated unit testing coverage
• ➖ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ➖ Documentation
• ➖ Code review by maintainers
• ➖ Full testing and approval
• ➖ Credit contributors
• ➖ Review with the product owner
• ➖ Release notes snippet
• ❌ Release

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A