Add a 'field_item_utility_classes' property to the field component

Problem/Motivation

In field component, 'field__item' class is hardcoded

Proposed resolution

In field.twig, add

{%
  set field_item_classes = [
    'field__item',
  ]|merge(field_item_utility_classes ?: [])
%}

and change the 2 lines with
<div {{ item.attributes.addClass('field__item') }}>{{ item.content }}</div>
to
<div {{ item.attributes.addClass(field_item_classes) }}>{{ item.content }}</div>
We can then use the component like this

{%
  include "radix:field" with {
    field_item_utility_classes: ['ratio', 'ratio-16x9'],
  }
%}

Remaining tasks

1. update field.twig
2. update field.component.yml
3. update documentation

MR is coming ;-)