Introduce decorate field option

[Paul-Bob]

Context

The format_using option is useful for formatting the output of a field, but it applies to both display and edit views. This often requires conditional logic to ensure proper behavior in different contexts. For example:

field :item, as: :text, format_using: -> {
  view.display? ? link_to(value, record.item_link, target: "_blank") : value
}

To simplify this, lets introduce a decorate option. This would function similarly to format_using, but only apply to display views (e.g., Show and Index), keeping the edit form unaffected.

With decorate, the same behavior could be achieved more cleanly:

field :item, as: :text, decorate: -> {
  link_to(value, record.item_link, target: "_blank")
}