volto Add support for custom item placeholder and fixed (non-add/removable) lists in ObjectListWidget

This PR implements two new features for the ObjectListWidget:

Custom item placeholder:
You can now define which field will be used as the placeholder/title for each item in the list, using the placeholderItemField prop. This makes it easier to identify items in large or complex lists. If placeholderItemField is not provided, the widget will fallback to the previous behavior, using the schema title or a default value.
Fixed (non-add/removable) lists:
The new fixedItems prop allows you to lock the list, disabling the add and remove buttons. This is useful for scenarios where the number of items should be fixed, but editing is still allowed. If fixedItems is not set, the widget keeps the previous behavior, allowing add and remove actions.

I'm open to suggestions for a better name for the fixedItems property!

Example usage

In the Teaser schema or any other, add just for testing purposes:

social_networks: {
  title: 'Redes Sociais',
  widget: 'object_list',
  fixedItems: true,
  placeholderItemField: 'name',
  schema: {
    title: 'Rede Social',
    fieldsets: [
      {
        id: 'default',
        title: 'Default',
        fields: ['name', 'url'],
      },
    ],
    properties: {
      name: {
        title: 'Nome',
        type: 'string',
        choices: [
          ['facebook', 'Facebook'],
          ['twitter', 'Twitter'],
          ['instagram', 'Instagram'],
          ['linkedin', 'LinkedIn'],
        ],
        default: 'facebook',
      },
      url: {
        title: 'URL',
        type: 'string',
        default: '',
      },
    },
    required: ['name', 'url'],
  },
  default: [
    {
      '@id': uuid(),
      name: 'facebook',
      url: 'https://facebook.com/exemplo',
    },
    {
      '@id': uuid(),
      name: 'twitter',
      url: 'https://twitter.com/exemplo',
    },
    {
      '@id': uuid(),
      name: 'instagram',
      url: 'https://instagram.com/exemplo',
    },
    {
      '@id': uuid(),
      name: 'linkedin',
      url: 'https://linkedin.com/in/exemplo',
    },
  ],
},